Electrowetting element with multiple electrodes

ABSTRACT

An electrowetting display device comprising an electrowetting element comprising a control system, a first fluid, a second fluid immiscible with the first fluid, and a first and second support plate. A first and second electrode are, respectively, overlapped by a first and second portion of a surface of the first support plate. The control system is operable to, in response to input data indicative of a first grey level, apply a first voltage between the second fluid and the first electrode such that the second fluid is in contact with at least part of the first portion and, subsequently, apply a second voltage between the second fluid and the second electrode to translate the first fluid such that the second fluid is in contact with at least part of the second portion.

BACKGROUND

Electrowetting display devices are known. In an off state of a knownelectrowetting element an oil layer covers a display area. In an onstate the oil layer is retracted so as to cover less of the displayarea. To switch the electrowetting element to the on state a voltage isapplied via an electrically conductive fluid immiscible with the oil. Toswitch the electrowetting element to the off state, the voltage isswitched off. By switching the oil and the electrically conductive fluidto different fluid configurations, different optical states can bedisplayed by the electrowetting element.

It is known to control the movement of the oil and the electricallyconductive fluid for example by providing a surface in theelectrowetting element with a different wettability for the oil than forthe electrically conductive fluid or by appropriately selecting theshape of the electrowetting element to cause a preferred direction ofmotion of the fluids upon application of a non-zero voltage.

It is desirable to provide improved techniques for fluid motion controlin an electrowetting element.

SUMMARY

In some embodiments, an electrowetting display device is provided. Thedevice includes an electrowetting element comprising a first fluid, asecond fluid immiscible with the first fluid, a first support platehaving a first surface and an opposing second surface, the first supportplate comprising at least one wall corresponding to a perimeter of thefirst surface, a first electrode overlapped by a first portion of thefirst surface, and a second electrode overlapped by a second portion ofthe first surface, with the first portion and the second portionnon-overlapping each other, and a second support plate, the first fluidand the second fluid located between the first support plate and thesecond support plate; and a control system operable to, in response toinput data indicative of a first grey level: apply a first voltagebetween the second fluid and the first electrode such that the secondfluid is in contact with at least part of the first portion; and,subsequently, apply a second voltage between the second fluid and thesecond electrode to translate the first fluid such that the second fluidis in contact with at least part of the second portion.

In some embodiments, a display apparatus is provided. The apparatusincludes an electrowetting element comprising a first fluid, a secondfluid immiscible with the first fluid, a first support plate comprisinga hydrophobic surface, at least one wall corresponding to a perimeter ofthe hydrophobic surface, a first electrode overlapped by a first portionof the hydrophobic surface, and a second electrode overlapped by asecond portion of the hydrophobic surface, with the first portion andthe second portion non-overlapping each other, and a second supportplate, the first fluid and the second fluid located between the firstsupport plate and the second support plate, at least one processor, andat least one memory comprising computer program instructions, the atleast one memory and the computer program instructions operable to, withthe at least one processor: receive input data indicative of a greylevel, in response to the input data: apply a sequence of voltagesbetween the second fluid and the first electrode and between the secondfluid and the second electrode to translate the first fluid across thehydrophobic surface such that the first fluid is in contact with atleast part of the first portion at a first time, the first fluid is incontact with at least part of the second portion at a second timesubsequent to the first time, and the first fluid is in contact with theat least part of the first portion at a third time subsequent to thefirst time and the second time.

In some embodiments, an electrowetting display device is provided. Thedevice includes an electrode, an electrowetting element comprising afirst fluid, a second fluid immiscible with the first fluid, a firstsupport plate having a hydrophobic surface, the first support platecomprising at least one wall corresponding to a perimeter of thehydrophobic surface, the at least one wall comprising a first wallportion, the first wall portion being substantially straight in a planeparallel to a plane of the hydrophobic surface, a second wall portion,the second wall portion being substantially straight in the planeparallel to the plane of the hydrophobic surface, and a third wallportion which connects the first wall portion to the second wallportion, the third wall portion being curved in the plane parallel tothe plane of the hydrophobic surface, a first electrode overlapped by afirst portion of the hydrophobic surface, and a second electrodeoverlapped by a second portion of the hydrophobic surface, with thefirst portion and the second portion non-overlapping each other, and asecond support plate, the first fluid and the second fluid locatedbetween the first support plate and the second support plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of an example electrowetting element;

FIG. 2 is a plan view of part of an example electrowetting displaydevice including the example electrowetting element of FIG. 1;

FIG. 3 shows schematically an example of circuitry for controlling anelectrowetting display device;

FIGS. 4a and 4b illustrate an example of different fluid configurationsfor the part of the electrowetting display device of FIG. 2;

FIGS. 5a to 5f illustrate a further example of different fluidconfigurations for the part of the electrowetting display device of FIG.2;

FIG. 6 is a plan view of part of a further example electrowettingdisplay device;

FIGS. 7a to 7g illustrate an example of different fluid configurationsfor the part of the electrowetting display device of FIG. 6;

FIG. 8 is a plan view of a part of a yet further example electrowettingdisplay device;

FIGS. 9a to 9c illustrate an example of different fluid configurationsfor the part of the electrowetting display device of FIG. 8;

FIGS. 10a to 10f illustrate an example of different fluid configurationsfor the part of the electrowetting display device of FIG. 8;

FIG. 11 is a plan view of part of a further example electrowettingdisplay device;

FIGS. 12a to 12c illustrate a yet further example of different fluidconfigurations for the part of the electrowetting display device of FIG.2; and

FIG. 13 shows a schematic system diagram of an example apparatusincluding an electrowetting display device.

DETAILED DESCRIPTION

Examples are described herein of an electrowetting element with a firstfluid, such as an oil, and a second fluid, such as an electrolyte, thatis immiscible with the first fluid. A support plate of theelectrowetting element for example includes two electrodes. In responseto data indicative of a grey level, for example corresponding to aparticular display effect for the electrowetting element to display, afirst voltage may be applied between one of the two electrodes and thesecond fluid and subsequently a second voltage may be applied betweenthe other of the two electrodes and the second fluid. In such examples,the first voltage may be applied to contract or retract the first fluidso that the first fluid is at least partly retracted from the surface ofthe electrowetting element. For example, the first fluid may be in theform of a droplet, blob or globule on the surface. The first fluid maythen be translated across the surface by application of the secondvoltage. Thus, by applying the first voltage and the second voltage, theposition of the first fluid for a particular grey level, or displaystate to be displayed by the electrowetting element, can be controlled.

In further examples of controlling an electrowetting element with asupport plate with two electrodes, a first voltage may be appliedbetween one of the two electrodes and the second fluid in response tofirst data indicative of a first grey level. A second voltage may thenbe applied between the other of the two electrodes and the second fluidin response to second data indicative of a second grey level. This canimprove the flexibility of control of the first fluid. For example,instead of motion of the first fluid initiating repeatedly at the sameinitiation location in the electrowetting element and the first fluidcollecting repeatedly at the same collection location (which istypically different from the initiation location), motion of the firstfluid may initiate at different positions or locations and the firstfluid may, similarly, collect at different locations over time. Thistherefore allows the first fluid motion to be controlled more flexibly,which may improve the display quality of the electrowetting element.

The structure of an example electrowetting element and its operation aspart of an electrowetting display device will first be described withreference to FIG. 1. Subsequently, the operation of an electrowettingdisplay device according to examples will be described in more detail.

FIG. 1 shows a diagrammatic cross-section of part of an example of anelectrowetting display device 1, which may be referred to as a displaydevice. The electrowetting display device 1 includes a plurality ofelectrowetting elements 2, which may otherwise for example be referredto as picture elements, display elements or electrowetting cells, one ofwhich is shown in the Figure. The lateral extent of the electrowettingelement is indicated in FIG. 1 by two dashed lines 3, 4. Theelectrowetting elements comprise a first support plate 5 and a secondsupport plate 6. The support plates may be separate parts of eachelectrowetting element or the support plates may instead be shared incommon by the plurality of electrowetting elements. The first supportplate 5 and the second support plate 6 may include first and secondsubstrates 7 a, 7 b, which may be made of glass or polymer. One or bothof the first and second support plates 5, 6 may be rigid or flexible.

The electrowetting display device 1 has a viewing side 8 on which animage or display effect formed by the display device can be viewed and arear side 9. In the Figure a side of the first support plate 5corresponds with the rear side 9 and a side of the second support plate6 corresponds with the viewing side 8. Alternatively, in other examples,a side of the first support plate may correspond with the viewing side.The display device may be of the reflective, transmissive ortransflective type. The display device may be an active matrix drivendisplay device. The plurality of electrowetting elements may bemonochrome. For a color display device the electrowetting elements maybe divided in groups, each group having a different color;alternatively, an individual electrowetting element may be able to showdifferent colors.

The second support plate 6 is positioned such that a first fluid 11 anda second fluid 12 are located between the first support plate 5 and thesecond support plate 6, in a space 10 between the first support plate 5and the second support plate 6, sometimes referred to as a cavity. Inthe example of FIG. 1, each electrowetting element 2 includes arespective portion of the first fluid 11 and the second fluid is sharedby the array of electrowetting elements. However, in other examples,each electrowetting element may include an individual or separateportion of the second fluid, for example where the second fluid is notshared by the array of electrowetting elements. At least one of thefirst and second fluids may be a liquid. The second fluid is immisciblewith the first fluid in examples such as that of FIG. 1. Therefore, thefirst fluid and the second fluid do not substantially mix with eachother and in some examples do not mix with each other to any degree. Theimmiscibility of the first and second fluids is due to the properties ofthe first and second fluids, for example their chemical compositions;the first and second fluids tend to remain separated from each other,therefore tending not to mix together to form a homogeneous mixture ofthe first and second fluids. Due to this immiscibility, the first andsecond fluids at least partially meet, for example contact, each otherat an interface labelled 55 in FIG. 1 for when no voltage is applied andlabelled 57 for when a voltage is applied, which interface typicallycorresponds to a boundary between the volume of the first fluid and thevolume of the second fluid; this interface or boundary may be referredto as a meniscus. With the first and second fluids substantially notmixing with each other, it is envisaged in some examples that there maybe some degree of mixing of the first and second fluids, but that thisis considered negligible in that the majority of the volume of firstfluid is not mixed with the majority of the volume of the second fluid.

The second fluid is electrically conductive or polar and may be water,or a salt solution such as a solution of potassium chloride in water. Inexamples, the second fluid is polar and in some examples is electricallyconductive, but in other examples is not electrically conductive.Electrically conductive typically refers to a second fluid capable ofconducting electricity for example. For example an electrical currentmay flow through the second fluid due to the flow of ions through thesecond fluid. In examples, a polar fluid, such as a polar second fluid,includes at least one compound (for example a liquid vehicle) having amolecule with a net dipole. Thus, across the molecular structure themolecule may have an overall dipole moment, due to an electrondistribution, with at least one part of the molecule having a negativeelectrical charge and at least one different part of the molecule havinga positive electrical charge. Such dipole moments may include permanentdipoles. The polarity is caused for example by the presence of one ormore atom to atom bonds in the molecule, with for example one of theatoms being a heteroatom such as oxygen or nitrogen. For example, such apolar atom to atom bond is a bond between an oxygen (O) atom and ahydrogen (H) atom, i.e. an —O—H bond. The second fluid may betransparent.

The first fluid is typically electrically non-conductive and may forinstance be an alkane like hexadecane or may be an oil such as siliconeoil. The first fluid is therefore non-polar in at least some examples.

The first fluid may absorb at least a part of the optical spectrum. Thefirst fluid may be transmissive for a part of the optical spectrum,forming a color filter. For this purpose the first fluid may be coloredby addition of pigment particles or a dye. Alternatively, the firstfluid may be black, for example absorb substantially all parts of thevisible spectrum, or reflecting. A reflective first fluid may reflectthe entire visible spectrum, making the layer appear white, or part ofit, making it have a color. In some examples, the first fluid is blackand therefore absorbs substantially all parts of the optical spectrum,for example in the visible light spectrum. In other examples, the firstfluid is another color than black and absorbs another part of theoptical spectrum, for example a sub-range of wavelengths within thevisible spectrum. In other examples, the display device includeselectrowetting elements having first fluids which are respectively red,green or blue, or cyan, magenta and yellow to provide a full colordisplay. Typically, by absorbing substantially all parts of the opticalspectrum, there is nonetheless a degree of variation, therefore thefirst fluid may not absorb all wavelengths, but the majority ofwavelengths within a given spectrum such as the visible spectrum, so asto perform the function of the first fluid in the electrowettingelement. The first fluid may therefore be configured to absorbsubstantially all light incident on the first fluid. For example thefirst fluid may absorb 90% or more of light in the visible spectrum andincident on the first fluid.

The first support plate 5 includes an insulating layer 13. Theinsulating layer 13 may be transparent or reflective. The insulatinglayer 13 may extend between walls 21 of an electrowetting element 2. Toavoid short circuits between the second fluid 12 and electrodes arrangedunder the insulating layer, layers of the insulating layer may extenduninterrupted over a plurality of electrowetting elements 2, as shown inFIG. 1. The insulating layer has a surface 14 facing the space 10 of theelectrowetting element 2. In this example the surface 14 is hydrophobic,although in other examples the surface may have different wettabilityproperties or may include a hydrophobic portion and a hydrophilic orless hydrophobic portion. The thickness of the insulating layer may beless than 2 micrometers and may be less than 1 micrometer. The surface14 is in contact with at least one of the first fluid or the secondfluid, depending on the fluid configuration as described below. Thesurface 14 may be considered to be a first surface or a top surface ofthe first support plate 5. The first support plate 5 also has anopposing second surface, which is on the opposite side of the firstsupport plate 5 than the first surface. The second surface may thereforebe considered to be a bottom surface of the first support plate 5 in theorientation illustrated in FIG. 1. For example, the bottom surface ofthe first support plate 5 may be the surface of the first substrate 7 athat is closest to or faces the rear side 9.

The insulating layer may be a hydrophobic layer; alternatively, it mayinclude a hydrophobic layer 15 and a dielectric layer 16 withpredetermined dielectric properties, the hydrophobic layer 15 facing thespace 10, as shown in FIG. 1 The hydrophobic layer is schematicallyillustrated in FIG. 1 and may be formed of Teflon® AF1600. Thedielectric layer 16 may have a thickness, taken in a directionperpendicular to a plane of the substrate, of between 5 nanometers andseveral micrometers, for example between 50 nanometers and 2 micrometersor 3 micrometers. In other examples the thickness may be between 50nanometers and 500 nanometers. The dielectric layer may be made of aninorganic material like silicon oxide or silicon nitride.

The hydrophobic character of the surface 14 causes the first fluid 11 toadhere preferentially to the insulating layer 13, since the first fluidhas a higher wettability with respect to the surface of the insulatinglayer 13 than the second fluid 12. Wettability relates to the relativeaffinity of a fluid for the surface of a solid. Wettability may bemeasured by the contact angle between the fluid and the surface of thesolid. The contact angle is determined by the difference in surfacetension between the fluid and the solid at the fluid-solid boundary. Forexample, a high difference in surface tension can indicate hydrophobicproperties.

The first fluid 11 in this example is substantially confined to anelectrowetting element by walls 21 that follow the cross-section of theelectrowetting element. For example, the first fluid 11 may remainwithin the electrowetting element 2 within operational tolerances. Insuch cases, a relatively small amount of the first fluid 11 maynevertheless transfer to an adjacent or neighboring electrowettingelement, for example by spilling over a wall 21 due to an impact on thefirst or second support plate or another shock, which may compress thefirst and second fluids 11, 12 and cause the first fluid 11 to beexpelled from the electrowetting element 2. The amount of first fluid 11that transfers in this way is generally sufficiently small to avoidadversely affecting the display quality or the contrast of theelectrowetting element 2. The extent of the electrowetting element,indicated by the dashed lines 3 and 4, is taken between the center ofthe walls. The area of the surface 14 between the walls of anelectrowetting element, indicated by the dashed lines 22 and 23, iscalled the display area 24, over which a display effect occurs.

In examples, the walls 21 may extend from the first support plate 5 tothe second support plate 6, or the walls may extend only partly from thefirst support plate 5 to the second support plate 6 as shown in FIG. 1.Although the walls 21 are shown as structures protruding from theinsulating layer 13 in the example of FIG. 1, in other examples thewalls may instead be formed by a surface layer of the support plate thatacts as a boundary to retain fluid 11 by repelling the first fluid. Forexample, walls 21 may be formed by providing a portion of the surface 14with a substantially different wettability. For example, the walls mayinclude a hydrophilic or less hydrophobic layer. In further examples,the walls may include any combination of materials, including materialsthat extend away from the first support plate and surface layers with asubstantially different wettability than a portion of the display area24.

The walls 21 may be a plurality of separate structures, for example thatare adjoining, in contact with each other or sufficiently close tosubstantially confine the first fluid 11 with the electrowetting element2. In other examples, though, the walls 21 may form a singlesubstantially continuous (for example continuous within manufacturingtolerances) or continuous structure per electrowetting element or acrossa portion or the entirety of the electrowetting display device 1.

An electrowetting element in examples such as that of FIG. 1 includes atleast one electrode as part of the first support plate 5. In the exampleof FIG. 1, the electrowetting element 2 includes a first electrode 17 aand a second electrode 17 b. A respective portion of a surface of theelectrowetting element, which in this example is the surface 14 of thehydrophobic layer 15 between the walls 21 of the electrowetting element2, overlaps each respective electrode. A first portion 18 a of thesurface 14 of the electrowetting element 2 overlaps the first electrode17 a and a second portion 18 b of the surface 14 of the electrowettingelement 2 overlaps the second electrode 17 b. The first portion 18 a andthe second portion 18 b are non-overlapping in examples. In other words,the first and second portions 18 a, 18 b are different areas or regionsof the surface, which do not coincide with or cover each other. However,the first and second portions 18 a, 18 b may be adjacent or neighboringeach other, without overlapping each other. In the example of FIG. 1,the first portion 18 a and the second portion 18 b together form theentirety of the surface 14 of the hydrophobic layer 15 but in otherexamples, the surface may include at least one other portion or regionthan the first portion 18 a and the second portion 18 b. The firstelectrode 17 a and the second electrode 17 b in FIG. 1 are smaller insize, for example occupying a smaller area in a plane of the firstsubstrate 7 a than the first portion 18 a and the second portion 18 brespectively. The first and second electrodes 17 a, 17 b arenevertheless entirely overlapped by the first and second portions 18 a,18 b respectively. However, in other examples, the first and secondelectrodes may not be completely overlapped by the first and secondportions 18 a, 18 b respectively. For example, instead, substantiallyall, such as 90% of, the first and second electrodes 17 a, 17 b may beoverlapped by the first and second portions 18 a, 18 b respectively, orthe first and second electrodes 17 a, 17 b may be overlapped by lessthan 90% of the first and second portions 18 a, 18 b. Furthermore, inexamples, the first electrode 17 a and the second electrode 17 b may bethe same size as, for example aligned with and completely overlapped by,the first and second portions 18 a, 18 b respectively, or differentsizes or misaligned with the first and second portions 18 a, 18 brespectively. In examples, a first electrode 17 a may be considered tobe overlapped by the first portion 18 a where the first portion 18 acovers, extends or lies over the first electrode 17 a or vice versa, andsimilarly for the second electrode 17 b and the second portion 18 n.

As will be appreciated by the skilled person, the arrangement of theelectrodes 17 a, 17 b in the example of FIG. 1 is not intended to belimiting. In examples, a first support plate of an electrowettingelement may include any number of electrodes, each associated with adifferent respective surface portion.

In examples, the first electrode 17 a may have substantially the sameoptical properties as the second electrode 17 b. For example, the firstelectrode and the second electrode 17 a, 17 b may both interact withlight, such as visible light, in substantially the same manner, forexample the same within measurement tolerances. Optical propertiestypically include at least one of a reflectivity, a transmissivity, anabsorptivity or other characteristic that governs or determines how amaterial interacts with electromagnetic radiation. For example, thefirst electrode and the second electrode 17 a, 17 b may be made from orinclude the same materials, or materials that have the same orsubstantially the same optical properties.

The first electrode 17 a may be reflective for light of at least onewavelength, for example a first wavelength, and the second electrode 17b may also be reflective for light of the at least one wavelength, forexample for the first wavelength. For example, both the first and secondelectrodes 17 a, 17 b may be reflective for light of substantially allwavelengths in the visible spectrum, for example a majority or allwavelengths in the visible spectrum or a sufficiently large number ofwavelengths to operate as a reflector in a reflective electrowettingelement 2. Alternatively, both the first and second electrodes 17 a, 17b may be reflective for light of a predetermined range of wavelengths inthe visible spectrum, such as a range of wavelengths corresponding to aparticular color, to create a colored display effect by reflection oflight from the first and/or second electrodes 17 a, 17 b.

In cases in which the first electrode 17 a and the second electrode 17 bare reflective, the first electrode 17 a may be arranged to reflectlight of the at least one wavelength, for example light of the firstwavelength, from first incident light incident on the first electrode 17a after transmission through the second support plate 6 and the secondelectrode 17 b may be arranged to reflect light of the at least onewavelength, for example light of the first wavelength, from secondincident light incident on the second electrode 17 b after transmissionthrough the second support plate 6. In other words, a first reflectivesurface of the first electrode 17 a may be positioned to reflect lightof the first wavelength from the first incident light incident on thefirst electrode 17 a after transmission through the second support plate6 and a second reflective surface of the second electrode 17 b may bepositioned to reflect light of the first wavelength from the secondincident light incident on the second electrode 17 b after transmissionthrough the second support plate 6. For example, one or both of thefirst reflective surface and the second reflective surface may face thesecond support plate 6 or the viewing side 9 of the electrowettingelement 2 as shown in FIG. 1, to receive ambient light entering theelectrowetting element from an external environment. In other examplesin which the viewing side and the rear side are reversed, the firstreflective surface and the second reflective surface may each face theviewing side, which may for example correspond with a side of the firstsupport plate 5 rather than a side of the second support plate 6.

In other examples, such as examples in which the electrowetting element2 is arranged for transmissive operation, the first electrode 17 a maybe transmissive for light of substantially all wavelengths in thevisible spectrum and the second electrode 17 b may also be transmissivefor light of substantially all wavelengths in the visible spectrum. Inyet further examples, such as examples in which the electrowettingelement 2 is a transflective electrowetting element, the first electrodeand the second electrode may have different optical properties than eachother. For example, one of the first electrode and the second electrodemay be reflective for light of at least one wavelength or forsubstantially all wavelengths in the visible spectrum and the other ofthe first electrode and the second electrode may be transmissive forlight of substantially all wavelengths in the visible spectrum.

The first and second electrodes 17 a, 17 b are electrically insulatedfrom the first and second fluids 11, 12 by the insulating layer 13 inthe example of FIG. 1. In some examples, further layers may be arrangedbetween the insulating layer 13 and the first and second electrodes 17a, 17 b. The first and second electrodes 17 a, 17 b are separated fromelectrodes of the first support plate 5 of neighboring electrowettingelements by an electrically non-conductive layer NCL in FIG. 1. In theexample of FIG. 1, the first electrode 17 a is also separated from thesecond electrode 17 b by an electrically non-conductive layer NCL.However, in other examples, the first electrode 17 a may be separatedfrom the second electrode 17 b by a different electrically insulativelayer. For example, the insulating layer 13 may extend between the firstelectrode 17 a and the second electrode 17 b to insulate the first andsecond electrodes 17 a, 17 b from each other.

The first electrode 17 a and the second electrode 17 b can be of anydesired shape or form. The first electrode 17 a and the second electrode17 b are supplied with voltage signals by first and second signal lines19 a, 19 b respectively, schematically indicated in FIG. 1.

A second signal line 20 is connected to an additional electrode 25 thatis in electrical contact with the electrically conductive or polarsecond fluid 12. The additional electrode 25 may be common to allelements, for example when they are fluidly interconnected by and sharethe second fluid 12, uninterrupted by walls. The electrowetting element2 can be controlled by a voltage V applied between the signal lines 19 aand 20 and/or between the signal lines 19 b and 20 and hence between thefirst electrode 17 a and the second fluid 12 and/or between the secondelectrode 17 b and the second fluid 12. The first and second electrodes17 a, 17 b may be coupled to a display driving system. For example, in adisplay device having the electrowetting elements arranged in a matrixform, the first and second electrodes 17 a, 17 b can be coupled to amatrix of control lines on the substrate 7 a via the first and secondsignal lines 18 a, 18 b. The signal line 20 may also be coupled to thedisplay driving system or to a separate or different display drivingsystem.

A display effect provided by the electrowetting element 2 may depend onan extent that the first fluid 11 and the second fluid 12 adjoin orcontact the surface corresponding with the display area, in dependenceon the magnitude of the applied voltage V described above. The magnitudeof the applied voltage V therefore determines the configuration of thefirst and second fluids within the electrowetting element. In otherwords, the display effect depends on the configuration of the first andsecond fluid in the electrowetting element, which configuration dependson the magnitude of the voltage applied between the electrodes of theelectrowetting element. For example, for controlling the configurationof the first and second fluids, a constant potential may be applied tothe additional electrode 25 in contact with the electrically conductiveor polar second fluid 12 and the magnitude of a potential applied to atleast one of the first electrode 17 a and the second electrode 17 b maybe controlled. The display effect gives rise to a display state of theelectrowetting element for an observer looking at the display device.When switching the electrowetting element from one fluid configurationto a different fluid configuration the extent of second fluid adjoiningthe display area surface may increase or decrease, with the extent offirst fluid adjoining the display area surface decreasing or increasing,respectively. Thus, the display effect may in effect be controlled bycontrolling a configuration of the first fluid (and consequently thesecond fluid).

In examples described herein, when a zero or substantially zero voltageis applied between the first electrode 17 a and the additional electrode25 and between the second electrode 17 b and the additional electrode25, for example when the electrowetting element is in an off state, thefirst fluid 11 forms a layer between the walls, as shown in FIG. 1 withthe reference numeral 55. Typically, substantially zero in examplesrefers to a voltage which is minimal, for example as close to zero thatthe first fluid adjoins a maximum extent of the display area 24.Application of a voltage will retract the first fluid, for exampleagainst a wall as shown by the dashed shape 57 in FIG. 1. Thecontrollable shape of the first fluid, in dependence on the magnitude ofthe applied voltage, is used to operate the electrowetting element toprovide a display effect over the display area 24. For example,switching the fluids to increase adjoinment of the second fluid 12 withthe display area 24 may increase the brightness of the display effectprovided by the element.

This display effect determines the display state of the electrowettingelement which an observer will see when looking towards the viewing sideof the display device. The display device is capable of providingdisplay states from black to white, including various intermediate greystates; in a color display device, the display state may also includecolor.

In examples such as that of FIG. 1, the electrowetting element 2includes a color filter 26 overlapping at least the first electrode 17 aand the second electrode 17 b. The color filter 26 may overlap or coverthe entire surface 14 or display area 24 of the electrowetting element 2or substantially all of the display area 24, for example 90% or more ofthe display area 24, or the color filter 26 may overlap a smallerportion of the display area 24 than 90%. There may be a single non-whitecolor filter per or corresponding to each electrowetting element 2. Insuch cases, each electrowetting element 2 may act as a color filter of aparticular color. In other examples, though, an electrowetting elementmay be associated with more than one non-white color filter or viceversa. For example, an electrowetting element may include a plurality ofcolor filters each of a different color from each other, such as a redcolor filter, a blue color filter and a green color filter. In theseexamples, each color filter may be considered to correspond to adifferent respective sub-pixel of the electrowetting element. The colorfilter 26 may be located in any suitable location within theelectrowetting element 2. For example, the color filter 26 may be alayer of the first support plate 5 or of the second support plate 6; inFIG. 2, the second support plate 6 includes the color filter 26. Wherethe first support plate 5 includes the color filter 26, the color filter26 may be located between a reflector (such as a reflective electrode)and the surface 14, which is adjoined by at least one of the first orsecond fluids 11, 12 in examples in which the electrowetting element 2operates in a reflective manner.

As the skilled person will appreciate, a color filter is typically oneor more layers of a material which are configured to filter lightincident on the filter. Indeed, a combination of layers in cross-sectionof an electrowetting element may together filter out light of one ormany wavelengths to provide an output color effect and thereby act as acolor filter. For example, the color filter may remove or filter out aportion of light entering the color filter. The light that is filteredout is for example of one or a band of many wavelengths and/or colors oflight. So, a color filter generally has a degree of transparency topermit light not removed by the color filter to be transmitted throughthe color filter. The filtering property of a color filter depends forexample on a material the color filter is formed of or includes.Examples of a material for forming the color filter include a resistmaterial such as the JSR OPTMER™ CR series, which are pigment dispersedphoto-resists.

The color filter 26 may be a non-switchable color filter; in otherwords, the color filter 26 may have a fixed shape so that a spatialconfiguration of the non-switchable color filter 26 is not changeable,for example is not switchable. Thus, the non-switchable color filter 26may be a non-fluid color filter. This may be contrasted with for examplethe first fluid described above, which may include a dye or pigment toact as a color filter which is switchable between different first fluidconfigurations. The color filter 26 may further contribute to a displayeffect provided by the electrowetting element 2, in addition to aconfiguration of the first and second fluids 11, 12.

FIG. 2 shows a plan view of part of the electrowetting display device 1of FIG. 1, illustrating the electrowetting element 2 of FIG. 1. Thelateral dimension of the electrowetting element 2 in FIG. 2,corresponding to the dashed lines 3 and 4 of the electrowetting element2 in FIG. 1, is indicated by the dashed line 27. Line 28 indicates theinner border of the walls 21 of FIG. 1; this line 28 is also the edge,for example a perimeter, of the surface 14 of the first support plate 5,which for example corresponds with the display area 24. The first andsecond portions 18 a, 18 b of the surface 14, which in this exampleoverlap the first and second electrodes 17 a, 17 b respectively, arealso labelled in FIG. 2.

The electrowetting display device 1 of FIGS. 1 and 2 includes aplurality of electrowetting elements. In FIG. 2, the electrowettingelement 2 illustrated in FIG. 1 is labelled, as is a furtherelectrowetting element 102. The further electrowetting element 102 inthis example is the same as the electrowetting element 2. Furtherlabelling of the further electrowetting element 102 is therefore omittedin FIG. 2, for clarity. The electrowetting elements may be arranged in amatrix configuration, for example as an array or matrix of n rows and mcolumns, where each of n and m are integers. Each of n and m may be ≥2;the total number of electrowetting elements in this example is n×m. Eachelectrowetting element of the matrix may be the same as each other orsome of the electrowetting elements may differ from each other.

In the example of FIG. 2, as in other examples, the first electrode 17 aand the second electrode 17 b are arranged in a row, with the firstelectrode 17 a adjacent to the second electrode 17 b. In such examples,the first electrode 17 a and the second electrode 17 b may be alignedwith each other, for example such that the first electrode 17 a and thesecond electrode 17 b lie on the same axis, for example an axis thatpasses through a center of the first and second electrodes 17 a, 17 balong a row of the electrowetting elements. The first and secondelectrodes 17 a, 17 b may be considered adjacent or neighboring eachother in examples in which there is no other electrode between the firstelectrode 17 a and the second electrode 17 b. There are typically othercomponents separating the first and second electrodes 17 a, 17 b though,such as the non-conductive layer NCL described above with reference toFIG. 1.

In examples such as FIG. 2, in which the electrowetting display deviceincludes a plurality of the electrowetting element 2, a first distancebetween the first electrode 17 a and the second electrode 17 b of afirst one of the electrowetting element 2 is smaller than a seconddistance between the first electrode 17 a of the first one of theelectrowetting element 2 and the first electrode 17 c of a second one ofthe plurality of the electrowetting element 2. In such examples, thefirst electrode 17 a neighbors the first electrode 17 c. In other words,electrodes within the same electrowetting element may be closer togetherthan neighboring electrodes in different, neighboring, electrowettingelements. This may increase or maximize a reflective area of theelectrowetting element 2, which can increase the contrast ratio of theelectrowetting element 2. Typically, the distance between electrodes inneighboring electrowetting elements depends on a thickness of the wallsbetween the neighboring electrowetting elements, as it may beundesirable to locate electrodes underneath the walls, as this may leadto undesirable reflections that may reduce the display quality of theelectrowetting element. For example, a distance between electrodes ofneighboring electrowetting elements may be the same as or larger than awall thickness in a plane of the surface 14, for example in a directionperpendicular to an inner surface of the wall, that faces towards acenter of the electrowetting element 2. However, in other examples, adistance between electrodes in neighboring electrowetting elements maybe independent of a wall thickness or a distance between neighboringelectrowetting elements (which may be considered to be aninter-electrowetting-element separation, for example).

At least one of: the first electrode of the first one of the pluralityof the electrowetting element, the second electrode of the first one ofthe plurality of the electrowetting element, the first electrode of thesecond one of the plurality of the electrowetting element, or the secondelectrode of the second one of the plurality of the electrowettingelement is reflective for light of at least one wavelength in examples.For example, as explained above, the first and second electrodes mayboth be reflective or may have substantially the same optical propertiesas each other and this may be the case for first and second electrodesof a plurality of electrowetting elements of the electrowetting displaydevice.

FIG. 3 shows schematically example circuitry 29 for use to control anexample electrowetting display device. The electrowetting display device1 of FIG. 3 may be similar to or the same as the electrowetting displaydevice 1 described above with reference to FIGS. 1 and 2; acorresponding description should be taken to apply. The electrowettingdisplay device 1 is for example a so-called active matrix drive typedisplay apparatus.

A control system, sometimes referred to as a display driving system, maybe used to control the electrowetting display device 1. The circuitry 29of FIG. 3 may be or form part of such a control system or displaydriving system. The control system in this example includes a displaycontroller or controller 30, a display row driver 32 and a displaycolumn driver 34. Data indicative of grey levels, which typicallycorrespond with display states of the electrowetting elements, thedisplay states for example representing a still image or video images,is received by the display controller 30 from an input line 36 to thedisplay driving system. The display controller 30 includes at least oneprocessor 38 for processing the data entered on the input line 36. Theat least one processor 38 is connected to at least one memory 40. Thedisplay controller 30 prepares the data for use in the electrowettingdisplay device 1 in this example.

The at least one memory 40 may store computer program instructions thatare configured to perform one or more of the methods of controlling anelectrowetting display device 1 described herein, when being executed bythe processor such as the at least one processor 38. For example, the atleast one memory 40 and the computer program instructions of the controlsystem may be operable to, with the at least one processor 38, apply afirst voltage between the second fluid 12 and the first electrode 17 aand, subsequently, apply a second voltage between the second fluid 12and the second electrode 17 b as described further below. The computerprogram instructions may be stored on a computer program productincluding a non-transitory computer-readable storage medium. Details ofthe at least one processor 38 and the at least one memory 40 aredescribed further below with reference to FIG. 13.

An output of the at least one processor 40 is connected by the line 42to the display row driver 32, which includes row driver stages 44 thattransform signals to the appropriate voltages for the electrowettingdisplay device 1. Row lines 46 connect the row driver stages 44 torespective rows of the electrowetting display device 1 for transmittingthe voltage pulses generated by the display row driver 32 toelectrowetting elements in each row of the electrowetting display device1, thereby providing a row addressing signal to each row of theelectrowetting display device. In other words, one or more voltagepulses for addressing one or more rows is transmitted over the row lines46 corresponding to the rows to switching elements correspondingrespectively to the electrowetting elements in the one or more rows. Thedisplay row driver 32 generates the voltage pulses used for addressingthe rows of the display device, using information from the at least oneprocessor 40 to set a value of the pulse duration of the voltage pulses.

Another output of the at least one processor 40 is connected by line 48to the display column driver 34, which includes column driver stages 52that transform signals to the appropriate voltages for theelectrowetting display device 1. Column lines 54 connect the columndriver stages 52 to the columns of the electrowetting display device 1,providing a column signal to each column of the electrowetting displaydevice 1. In the example of FIG. 3, the first electrode 17 a and thesecond electrode 17 b lie in different columns, for example in adjacentor neighboring columns. Thus, in this example, each electrowettingelement 2 is connected to two different column lines 54, one connectedto the first electrode 17 a and one connected to the second electrode 17b. However, in other examples, the first and second electrodes 17 a, 17b may be connected differently to the display column driver 34 or to thecolumn driver stages 52.

The display controller 30 determines which rows are selected foraddressing and in which order. The selected rows are consecutivelyaddressed by applying an addressing signal to each of these rows. Theaddressing may include the steps of determining a value for a firstpulse duration corresponding to at least one voltage pulse to be appliedto a row of electrowetting elements, generating the at least one voltagepulse having the first pulse duration and transmitting the at least onevoltage pulse to the rows to be addressed. In examples where theelectrowetting elements of a row are connected to the same row line,addressing a row for example refers to addressing one or more, forexample each, electrowetting element of that row. When an electrowettingelement is being addressed, the electrowetting element admits the columnsignal that is applied to the column line to which the electrowettingelement is connected. The column signal for an electrowetting element isapplied substantially simultaneously with the voltage pulse used foraddressing the electrowetting element. The column signal may beconsidered to be applied substantially simultaneously with the voltagepulse for example where the column signal is present on the column linefor at least the pulse duration of the voltage pulse.

In other examples, a column addressing signal may be applied to one ormore, for example, each column of the display device to admit a signallevel of a row signal to the first electrode 17 a or the secondelectrode 17 b. In other words, the functions of the display row driverand display column driver may be swapped, with the display column driverused to generate a voltage pulse for addressing columns of the displaydevice, for example to switch a transistor of each of the electrowettingelements of the column to a conducting state to pass the signal level ofthe display row driver to the electrowetting element electrode to setthe corresponding electrowetting element in a desired display state. Insuch cases, the first electrode 17 a and the second electrode 17 b maybe located in different rows from each other, for example on neighboringrows.

The display drivers may include a distributor, not shown in FIG. 3, fordistributing data input to the display driver over a plurality ofoutputs connected to the driver stages. The distributor may be a shiftregister. FIG. 3 shows the lines only for those columns and rows of theelectrowetting display device that are shown in the Figure. The displayrow drivers may be integrated in a single integrated circuit. Similarly,the display column drivers may be integrated in a single integratedcircuit. The integrated circuit may include the complete driverassembly. The integrated circuit in examples, such as that of FIG. 3, isintegrated on the first support plate 5, although in other examples theintegrated circuit may be integrated on the second support plate 6instead. The integrated circuit may include the entire display drivingsystem. Such an arrangement may be known as a “chip on glass” (COG)construction. In other examples a “chip on foil” (COF) construction maybe used, where the display column drivers and/or the display row driversmay be integrated on a foil, which is then arranged on the first orsecond support plate 5, 6, which foil is connectable to circuit lines ofthe first or second support plate 5, 6 for driving the electrowettingelements. The integrated circuit may include part or the entire controlsystem of the electrowetting display device 1.

In this example, the electrowetting elements of the electrowettingdisplay device 1 are arranged in a matrix with an active matrixconfiguration. FIG. 3 shows electrowetting elements for five rows,labelled k to k+4 and four columns labelled l to l+3. The total numberof rows and columns for common display devices may range between a fewhundred and a few thousand. The electrowetting elements of column l arelabelled p to p+4. Each electrowetting element may have the sameconstruction as the electrowetting element 2 in FIG. 1. In this example,each electrowetting element occupies two columns and one row, with eachcolumn corresponding to a different electrode of the first and secondelectrodes 17 a, 17 b of the first support plate 5. The electrowettingelement 2 of FIG. 2 is located in the row k+1 and columns l and l+1 inFIG. 3. Column l is associated with the first electrode 17 a and columnl+1 is associated with second electrode 17 b. With this arrangement, adifferent potential can be applied to the first electrode 17 a and thesecond electrode 17 b when the row k+1 is addressed, as the firstelectrode 17 a and the second electrode 17 b are located in the same rowof the matrix of electrowetting elements.

As noted above, FIG. 3 shows a few electrical parts of theelectrowetting elements. Each electrowetting element 2 of theelectrowetting display device 1 includes at least one active element inthe form of a switching element. The at least one switching element ofan electrowetting element 2 is not necessarily located within thelateral extent of the electrowetting element (for example between thedashed lines 3, 4 as shown in FIG. 1), although it may be. For example,there may be a first switching element connected to the first electrode17 a of the electrowetting element 2 and a second switching elementconnected to the second electrode 17 b of the electrowetting element 2.Nevertheless, if the respective switching element is connected to thefirst electrode 17 a or the second electrode 17 b of an electrowettingelement, directly or indirectly, the respective switching element may beconsidered to be included in that electrowetting element. A switchingelement may be a transistor, for example a thin-film transistor (TFT),or a diode.

The electrodes of the electrowetting element 2 are indicated as twoelectrowetting element capacitors Cp formed, respectively, by the firstelectrode 17 a and the additional electrode 25 and by the secondelectrode 17 b and the additional electrode 25. The first and secondelectrodes 17 a, 17 b may therefore each be considered to correspond toa first plate of a different respective electrowetting element capacitorCp and the additional electrode 25 may be considered to correspond to asecond plate of one of the two electrowetting element capacitors Cp. Aline connecting the additional electrode 25 of an electrowetting elementcapacitor Cp to a common potential, in this example ground, is thecommon signal line 20. A line connecting the first electrode 17 a of anelectrowetting element capacitor Cp to the transistor is the firstsignal line 19 a shown in FIG. 1 and a line connecting the secondelectrode 17 b of a different electrowetting element capacitor Cp to thetransistor is the second signal line 19 b shown in FIG. 1. Thus, inexamples, the control system may include first circuitry to apply afirst voltage between the second fluid 12 and the first electrode 17 aand second circuitry, different from the first circuitry, to apply asecond voltage between the second fluid 12 and the second electrode 17b. The first circuitry may for example include the first switchingelement, and the column driver stage 52 and column line 54 associatedwith the column of the first electrode 17 a. Similarly, the secondcircuitry may include the second switching element, and the columndriver stage 52 and column line 54 associated with the column of thesecond electrode 17 b.

The electrowetting element may include a storage capacitor Cs forstorage purposes or for making the duration of the holding state or thevoltage applied to the electrowetting element uniform across theelectrowetting display device. The storage capacitor Cs may be arrangedin parallel with Cp and is not separately shown in FIG. 3. A first plateof the storage capacitor Cs may be connected to a storage control lineto which a potential Vstorage is applied and a second plate of thestorage capacitor Cs may be connected to the first or second switchingelement connected to the first or second electrodes 17 a, 17 brespectively. For example both the first or second electrodes 17 a, 17 band the second plate of the storage capacitor Cs may be connected to adrain of a TFT.

As explained above, in examples, the display column driver 34 providesthe signal levels corresponding to the input data for the electrowettingelements. The display row driver 32 provides the signals for addressingthe row of which the electrowetting elements are to be set in a specificdisplay state. In examples, addressing a row for example refers toapplying a signal on the signal line of the row that switches atransistor of each of the electrowetting elements of the row to aconducting state of the transistor. Each row of the n rows of thedisplay device is addressable by a signal such as a voltage pulse; thevoltage pulse is applied to a switching element of each of theelectrowetting elements (or to a switching element associated with arespective electrode of an electrowetting element if there are aplurality of electrodes of the electrowetting element), in the addressedrow for switching the switching element.

The addressing of rows is part of the addressing of electrowettingelements in an active matrix display device. A specific or a specificelectrode of an electrowetting element is addressed by applying avoltage to the column in which the specific electrowetting element orelectrode of the electrowetting element is located and applying avoltage pulse to the row in which the specific electrowetting element islocated.

When the transistor of an electrowetting element receives at its gate avoltage pulse of its row addressing signal, the transistor becomesconducting and it passes the signal level of its display column driverto the first electrode 17 a or the second electrode 17 b of theelectrowetting element 2 (depending on whether the column includes thefirst electrode 17 a or the second electrode 17 b), and to the secondplate of the storage capacitor Cs in examples with a storage capacitorCs. In examples, a voltage pulse is a rapid, transient change in thevoltage from a baseline value to a higher or lower value, followed by arapid return or change to the baseline value. The time period betweenthe two subsequent voltage changes of the voltage pulse is called apulse duration. After the transistor has been switched off, so thetransistor is no longer conducting, the voltage over the cell will besubstantially maintained until the transistor is switched on again bythe next row addressing signal for the electrowetting element or for therespective electrode of the electrowetting element. The time duringwhich the transistor is switched off may be referred to in examples as aholding state. In this active matrix driving method the electrodes ofthe electrowetting elements are connected to the driver stages brieflyat the start of a period during which they show a certain grey level ordisplay effect. During this connection, a voltage related to the desiredgrey level or display effect is applied between the first electrode 17 aand the additional electrode 25 and/or between the second electrode 17 band the additional electrode 25. After the electrowetting element 2 isdisconnected from the driver stage, the voltage between the firstelectrode 17 a and the additional electrode 25 and/or between the secondelectrode 17 b and the additional electrode 25 is substantiallymaintained by one or more capacitors during the period during which theelectrowetting element shows the grey level or display effect, forexample by one or both of the two electrowetting element capacitors Cpand/or by the storage capacitor Cs. The period during which the voltageis substantially maintained is determined in these examples by thecombined capacitance and leakage currents of the capacitors. By using astorage capacitor Cs as well as the two electrowetting elementcapacitors Cp, the voltage may be substantially maintained for a longerduration than otherwise, although in other examples the electrowettingelement need not include a storage capacitor Cs. A voltage may beconsidered to be substantially maintained for example where a change inthe voltage is sufficiently small that it does not cause a visiblechange in a display state or grey level of an electrowetting element 2.For example, a change in display effect, for example transmission orreflectance, of less than 10% is typically not visible to a viewer. Themethod is called ‘active’, because the electrowetting element containsat least one active element, for example a transistor.

The example circuitry 29 of FIG. 3 may be used to control theelectrowetting element 2 of FIGS. 1 and 2 as described with reference toFIGS. 4a and 4b and FIGS. 5a to 5f , for example. As will be appreciatedby the skilled person, the example circuitry 29 of FIG. 3 may also beadapted for use with electrowetting elements with a first support plateincluding more than two electrodes, such as the electrowetting elements202, 302 of FIGS. 6 and 8. In such cases, each electrode may be locatedin a different column or row from each other and may have appropriatecircuitry to receive a respective potential from the display columndriver 34 or the display row driver 32, similar to that described abovefor the first and second electrodes 17 a, 17 b.

FIGS. 4a and 4b and FIGS. 5a to 5f illustrate different fluidconfigurations that may be obtained using the example electrowettingelement 2 of FIGS. 1 and 2. FIGS. 4a and 4b and FIGS. 5a to 5f show thesame part of the electrowetting display device 1 illustrated in FIG. 2,with corresponding elements labelled with the same reference numerals.Some of the reference numerals included in FIG. 2 are omitted in FIGS.4a and 4b and FIGS. 5a to 5f , for clarity.

FIGS. 4a and 4b show an example series of fluid configurations that maybe obtained by the first fluid and the second fluid in response to inputdata indicative of a first grey level. The first grey level for examplecorresponds to one of a series of levels, which may be predeterminedlevels, between a minimum grey level, which may for example correspondto lightest display state with the first fluid 11 adjoining orcontacting a minimum area of the surface, and a maximum grey level,which may for example correspond to a darkest display state, for examplewith the first fluid 11 spread out to cover the surface 14 of the firstsupport plate 5. For example, the minimum grey level may be consideredto correspond to a white display effect (although in practice thedisplay effect may be light grey rather than white) and the maximum greylevel may be considered to correspond to a black display effect.

Prior to FIG. 4a , the first fluid 11 may for example be in aconfiguration corresponding to the maximum grey level or darkest displaystate. In the example of FIGS. 4a and 4b , the control system of theelectrowetting display device 1 is configured to apply a first voltageand, subsequently, a second voltage in response to input data indicativeof a first grey level, which is for example a grey level other than themaximum grey level, such as a grey level corresponding to aconfiguration of the first and second fluids 11, 12 with the first fluid11 partly retracted from the surface so that both the first and secondfluids 11, 12 contact the surface.

Thus, in response to the input data, a first voltage is applied betweenthe second fluid 12 and the first electrode 17 a to configure the firstfluid 11 and the second fluid 12 in a first configuration with thesecond fluid in contact with at least part of the first portion 18 a,which overlaps the first electrode 17 a. This is illustrated in FIG. 4a. Thus, with the first voltage applied, the first fluid 11 in thisexample retracts or contracts away from an area of the surfacecorresponding to the first electrode 17 a (which in this example is thefirst portion 18 a of the surface). This is due to a larger electricfield between the first electrode 17 a and the second fluid 12, whichcauses motion of the first fluid 11 in contact with the first portion 18a to initiate before motion of the first fluid 11 in contact with thesecond portion 18 b due to the preferential adherence of the secondfluid 12 to the first portion 18 a with a voltage applied across thefirst portion 18 a. Accordingly, the direction of motion of the firstfluid 11 can be controlled, for example so that the first fluid 11retracts to contact the second portion 18 b.

In FIG. 4a and the other Figures described herein, the first fluid 11 isillustrated with a curved leading edge, with the upper and lower edgesof the first fluid 11 in the Figures extending further along the wallsthan the central portion of the first fluid 11. This may for example bedue to surface tension or capillary action acting on the first fluid 11.In other examples, the leading edge of the first fluid 11 may be of adifferent shape, though. For example, the leading edge of the firstfluid 11 may be substantially straight, for example straight withinmeasuring uncertainties, in a central region, and curved at the edgesclosest to the walls, such as with a curve similar to that shown for theupper and lower edges of the first fluid 11 in FIG. 4a . Furthermore, inFIG. 4a and the other Figures referred to herein, the first fluid 11 isshown with a leading edge that coincides with the edge of a non-activeelectrode. For example, in FIG. 4a , a higher voltage is applied betweenthe first electrode 17 a and the second fluid 12 than between the secondelectrode 17 b and the second fluid 12. Hence, the second electrode 17 bmay be considered to be a non-active electrode in this example. In FIG.4a , the leading edge of the first fluid 11 approximately aligns with oroverlaps the edge of the second electrode 17 b closest to the firstelectrode 17 a. This is due to the electric field between the firstelectrode 17 a and the second fluid 12 extending beyond the edge of thefirst electrode 17 a, in this example extending to the edge of thesecond electrode 17 b. This causes the first fluid 11 to retract beyondthe edge of the first electrode 17 a, to approximately coincide with theedge of the second electrode 17 b. As the skilled person willappreciate, though, the Figures are merely illustrative of examples. Inother examples, the leading edge of the first fluid 11 may be at adifferent position with respect to the first electrode 17 a than in FIG.4a . For example, if the voltage between the first electrode 17 a andthe second fluid 12 is lower than in the example of FIG. 4a (but stillhigher than a voltage between the second electrode 17 b and the secondfluid 12), the electric field may not extend as far as in FIG. 4a . Inthis case, the leading edge of the first fluid 11 may be closer to theedge of the first electrode 17 a than in the example of FIG. 4a and maynot coincide with the edge of the second electrode 17 b.

With the first voltage applied between the second fluid and the firstelectrode 17 a, a voltage between the second fluid and the secondelectrode 17 b has a smaller magnitude than the first voltage in thisexample. This therefore creates a larger electric field above the firstelectrode 17 a than above the second electrode 17 b, causing the firstfluid 11 to translate from contact with the first portion 18 a tocontact the second portion 18 b, as explained above. For example, amagnitude of the voltage between the second fluid and the secondelectrode 17 b may be zero or substantially zero (such as zero withinmeasurement errors) while the first voltage is applied.

In FIG. 4b , a second voltage is applied between the second fluid 12 andthe second electrode 17 b to translate the first fluid by configuringthe first fluid 11 and the second fluid 12 in a second configurationwith the second fluid 12 in contact with at least part of the secondportion 18 b. Translation of the first fluid 11 for example involvesmovement or motion of the first fluid 11 across or over the surface, forexample so that a line along which a boundary between the first fluidand the second fluid meets the surface (sometimes referred to as athree-phase line) also moves across the surface. Translation maytherefore involve shifting of the first fluid. Typically, both a leadingedge and a trailing edge of the first fluid 11 (in the direction ofmotion) move during translation of the first fluid 11. In contrast,during retraction or contraction of the first fluid 11, one of theleading edge or the trailing edge of the first fluid 11 may remain inthe same position and may be considered a static edge of the first fluid11, with the other of the leading edge or the trailing edge movingtowards the static edge to compress the first fluid, so that the firstfluid 11 adjoins or contacts a progressively smaller area of thesurface. Similarly, during expansion of the first fluid 11, one of theleading edge or the trailing edge of the first fluid 11 may remain inthe same position and may be considered a static edge of the first fluid11, with the other of the leading edge or the trailing edge moving awayfrom the static edge to expand the first fluid, so that the first fluid11 adjoins or contacts a progressively larger area of the surface.However, in some cases, translation may also involve contraction orexpansion of the first fluid 11. In these cases, a leading edge of thefirst fluid may move to a larger or greater extent, for example with alarger distance between a final position of the leading edge and aninitial position of the leading edge, than a trailing edge of the firstfluid (if the first fluid 11 expands) or vice versa (if the first fluid11 contracts). This may therefore cause a change in the extent or areaof contact between the first fluid 11 and the surface.

With the second voltage applied between the second fluid and the secondelectrode 17 b to translate the first fluid 11, a voltage between thesecond fluid and the first electrode 17 a may have a smaller magnitudethan the second voltage in examples. As described above for translationof the first fluid 11 to contact the second portion 18 b, this creates alarger electric field above the second electrode 17 b than above thefirst electrode 17 a, causing the first fluid 11 to translate fromcontact with the second portion 18 b to contact the first portion 18 a.For example, a magnitude of the voltage between the second fluid and thefirst electrode 17 a may be zero or substantially zero (such as zerowithin measurement errors) while the second voltage is applied. Forexample, the control system in examples such as that of FIGS. 4a and 4bmay be operable to, in response to the input data indicative of thefirst grey level, cease or withhold application of the first voltagebetween the second fluid 12 and the first electrode 17 a before or atsubstantially the same time (for example at the same time withinmeasurement uncertainties or while the row corresponding to the secondelectrode 17 b is being addressed) as application of the second voltagebetween the second fluid 12 and the second electrode 17 b. This mayfurther improve the control of the first and second fluids, for exampleso that the first fluid 11 is translated as explained above. Similarly,the control system may be operable to cease or withhold application ofthe second voltage between the second fluid 12 and the second electrode17 b before or at substantially the same time as application of thefirst voltage between the second fluid 12 and the first electrode 17 a.

In examples such as that of FIGS. 4a and 4b , the first fluid 11 is incontact with at least part of the second portion 18 b with the firstfluid 11 and the second fluid 12 in the first configuration, and thefirst fluid 11 is in contact with at least part of the first portion 18a with the first fluid 11 and the second fluid 12 in the secondconfiguration. However, in other examples, the first fluid may be in adifferent location, position or configuration with the first and secondfluids 11, 12 in the first or second configurations. Some of these otherexamples are described further below.

In the example of FIGS. 4a and 4b , the first and second fluids 11, 12are in at least two different configurations or positions for the samegrey level. Thus, in examples such as this, there may not be a singlepredefined or predetermined configuration for a particular grey level.Instead, there may be a one-to-many relationship between grey level andfluid configuration. This provides flexibility for control of thefluids. For example, the control system may be operable to apply asequence of voltages to configure the first fluid to a plurality ofdifferent configurations, each of the plurality of differentconfigurations respectively corresponding to the same grey level. Thesequence of voltages may include a first voltage between the secondfluid and the first electrode 17 a and a second voltage between thesecond fluid and the second electrode 17 b. An extent or area of contactbetween the first fluid and the surface may be substantially the same,for example the same or the same within measurement errors, for each ofthe plurality of different configurations. However, a location ofcontact between the first fluid and the surface may be different for atleast two of the plurality of different configurations. In other words,the first fluid may translate over the surface 14 without substantiallyexpanding or contracting to contact different locations of the surfacein the at least two of the plurality of difference configurations.

In examples, the control system may be operable to receive first inputdata indicative of a first grey level and, in response to the firstinput data, apply a voltage, which may for example be a third voltagebetween the second fluid and the second electrode, the third voltagecorresponding to the first grey level, or a fourth voltage between thesecond fluid and the second electrode, the fourth voltage correspondingto the first grey level. Alternatively, the control system may beoperable to apply a sequence of voltages corresponding to the first greylevel to the electrowetting element in response to the first input data,such as the voltages described above with reference to FIGS. 4a and 4b .Subsequently, the control system may be operable to receive second inputdata indicative of a second grey level different from the first greylevel and select one voltage of a sequence of voltages for applicationto the electrowetting element consecutively after application of thevoltage corresponding to the first grey level. For example, the controlsystem may be operable to select the one voltage of the sequence ofvoltages in dependence on a configuration of the first fluid with thevoltage corresponding to the first grey level applied. For example, inresponse to application of the fourth voltage corresponding to the firstgrey level between the second fluid and the second electrode 17 b, theone voltage selected for application consecutively after the voltagecorresponding to the first grey level may be the first voltage forapplication between the second fluid and the first electrode 17 a.Conversely, in response to application of the third voltagecorresponding to the first grey level between the second fluid and thefirst electrode 17 a, the one voltage may be the second voltage forapplication between the second fluid and the second electrode 17 b. Inother words, the one voltage may be selected to cause the first fluid 11to be translated from a configuration or position prior to theapplication of the one voltage, rather than to cause the first fluid 11to expand or contract without translation. For example, the firstvoltage may be selected in response to the fourth voltage being appliedbetween the second fluid and the second electrode 17 b and appliedconsecutively after the fourth voltage. In contrast, the second voltagemay be selected in response to the third voltage being applied betweenthe second fluid and the first electrode 17 a and applied consecutivelyafter the third voltage. Application of two voltages consecutively forexample refers to application of the two voltages one immediately afterthe other, with no intervening voltages applied therebetween.

The control system may be operable to, in response to the input dataindicative of the first grey level, and during display of the first greylevel by the electrowetting element 2, apply a sequence of voltagesincluding the first voltage and the second voltage to translate thefirst fluid 11 substantially continuously across the surface 14. In theexample of FIGS. 4a and 4b , the first fluid 11 may be movedsubstantially continuously backwards and forwards across the surface 14,so that the first fluid 11 repeatedly switches from contacting at leastpart of the second portion 18 b to contacting at least part of the firstportion 18 a. For example, the control system may be operable to causereciprocating motion of the first fluid across the surface.Reciprocating motion, sometimes referred to as reciprocal motion, is forexample a repetitive back-and-forth movement, typically linearly acrossthe surface from one side of the surface to an opposite side of thesurface. For example, the first fluid 11 may be moved repeatedly in alinear direction between at least partial contact of the second portion18 b and at least partial contact of the first portion 18 a.Substantially continuous translation may refer to motion of the firstfluid that is either continuous or unceasing or that halts or stops butfor a relatively short time period such as a time period that would beimperceptible to a viewer or to a time period that is short, for example10% or less, compared with the time period during which a display statecorresponding to the first grey level is to be maintained by theelectrowetting element 2 of the electrowetting display device 1. Forexample, the first fluid 11 may be moved substantially continuouslythroughout the display of the first grey level by application of thesequence of voltages, for example for all or substantially all, such asat least 90%, of a time period over which the first grey level isdisplayed.

By translating the first fluid continuously or substantiallycontinuously in response to input data corresponding to a first greylevel, the first fluid may be more likely to group together as a singleportion or as a smaller number of portions than otherwise. This mayimprove the quality of the display effect obtained by the electrowettingelement, for example by improving contrast.

For example, where the first fluid is colored, portions of the firstfluid in the display area of the electrowetting element, e.g. due tobreak-up of the first fluid layer when a voltage is applied between thefirst electrode and the second fluid or between the second electrode andthe second fluid, can reduce the transmission of light through thedisplay area of the electrowetting element, reducing the display qualityof electrowetting element. For example, the display effect obtained witha particular voltage applied to the electrode of the electrowettingelement may be darker than intended if first fluid portions are presentin the display area, as the light intended to pass through the displayarea, e.g. from a backlight or from ambient light, is obscured by thecolored first fluid portions. This can therefore lead to an incorrectbrightness or greyscale being displayed by the electrowetting element.Moreover, it may no longer be possible obtain the whitest or lightestdisplay state, e.g. with the first fluid maximally retracted, as even ifa voltage corresponding to such a display state or grey level isapplied, portions of the first fluid in the display area would tend tomake the display state or grey level appear darker, e.g. with reducedtransmission of light through the electrowetting element. Thus, suchfirst fluid portions may reduce the range of display states or greylevels obtainable by the electrowetting element, for example as thelightest state or minimum grey level may be darker than for anelectrowetting element which does not suffer from first fluid portionsin the display area.

For example, a small portion, droplet or blob of first fluid, sometimesreferred to as a satellite, may separate or break up from a largerportion of the first fluid and remain on the first portion 18 a of thesurface as the first fluid moves to contact the second portion 18 b ofthe surface. However, as in examples the first fluid 11 is controlled toreturn to its original position or to contact the first portion 18 a ofthe surface as it did previously, the first fluid 11 may collect orcoalesce with the remaining small portion on the first portion 18 a, toreduce the number of separate portions of first fluid from two to one.This may therefore improve the contrast or display quality as explainedabove.

In other examples, the sequence of voltages may be applied to obtainreciprocating motion of the first fluid for less than an entire durationof display of a grey level. For example, the sequence of voltages may beused to cause the first fluid to move back and forth a limited number oftimes. There may then be a pause in motion of the first fluid, forexample with the first fluid coming to rest or being stationary, forexample before motion of the first fluid resumes or before the firstfluid is switched or configured in a configuration corresponding to adifferent grey level.

In examples as FIGS. 4a and 4b , the control system may therefore beconfigured to, in response to input data indicative of a grey level suchas the first grey level, apply a sequence of voltages between the secondfluid and the first electrode and between the second fluid and thesecond fluid to translate the first fluid across the surface, which isfor example a hydrophobic surface, such that the first fluid is incontact with at least part of the first portion at a first time, thefirst fluid is in contact with at least part of the second portion at asecond time subsequent to the first time and the first fluid is incontact with the at least part of the first portion at a third timesubsequent to the first time and the second time. For example, the firsttime, the second time and the third time may be consecutive to eachother. In such cases, there may be no intervening voltages between thevoltages to obtain, respectively, the configuration of the first fluidin contact with the at least part of the first portion at the firsttime, the at least part of the second portion at the second time and theat least part of the third portion at the third time.

Control systems in examples such as FIGS. 4a and 4b may further beoperable to or configured to apply a sequence of voltages to translatethe first fluid over the surface during display of a first grey level,the sequence of voltages including a first voltage between the secondfluid and the first electrode and a second voltage between the secondfluid and the second electrode. For example, the sequence of voltagesmay be applied throughout or while the first grey level is beingdisplayed, either for the entirety of or substantially all of (such asat least 90% of) a time during which the first grey level is to beapplied or during a portion of the display of the first grey level.

For example, the sequence of voltages may be used to translate the firstfluid from being at least partly in contact with the first portion (forexample, in a retracted configuration, prior to the configurationillustrated in FIG. 4a ) to being at least partly in contact with thesecond portion (as shown in FIG. 4a ) to, subsequently or after the atleast partial contact with the second portion, being at least partly incontact with the first portion (as shown in FIG. 4b ). In the example ofFIGS. 4a and 4b , the control system is operable to apply the sequenceof voltages to translate the first fluid from being at least partly incontact with the first portion but not substantially in contact with thesecond portion, to being at least partly in contact with the secondportion but not substantially in contact with the first portion, forexample in contact with zero, zero within measurement uncertainties orless than 10% of the first portion, to, subsequently, being at leastpartly in contact with the first portion but not substantially incontact with the second portion, for example in contact with zero, zerowithin measurement uncertainties or less than 10% of the second portion.

In examples such as that of FIGS. 4a and 4b , the first fluid may be incontact with a first extent or area of the surface, with the first fluidand the second fluid in the first configuration, and the first fluid maybe in contact with a second extent or area of the surface, with thefirst fluid and the second fluid in the second configuration, the secondextent substantially the same size as or smaller than the first extent.For example, a sequence of voltages including the first voltage and thesecond voltage may be applied to translate the first fluid across thesurface with a substantially constant extent or area of contact betweenthe first fluid and the surface. In other words, the first fluid may betranslated or moved across the surface by the application of the firstand second voltages, without the first fluid substantially or noticeablycontracting or expanding. This may allow the grey level displayed by theelectrowetting element 2 to be maintained during the application of thefirst and second voltages. By maintaining a display effect or grey levelof the electrowetting element 2 in this way, the effect of backflow, inwhich the first fluid has a tendency to flow back to cover the surfaceof the electrowetting element 2 despite a voltage being maintained, maybe reduced or eliminated.

The control system may be operable to, in response to the input dataindicative of the first grey level, apply a sequence of voltagescomprising repeated application of the first voltage between the secondfluid and the first electrode and, subsequently, the second voltagebetween the second fluid and the second electrode. For example, thesequence of voltages may be a series or succession of voltages that areapplied sequentially, consecutively or one after the other. In examples,the first voltage and the second voltage may alternate; for example, thefirst voltage may first be applied, the second voltage may be appliedafter the first voltage has been applied, and after the second voltagehas been applied, the first voltage may be applied again, and so on. Insuch cases, the second voltage may be removed or a magnitude of thesecond voltage may be altered while the first voltage is applied andvice versa.

In other examples, though, the sequence may be a non-repeating sequenceor may include non-repeating voltages. For example, a duration ofapplication and/or a magnitude of one or both of the first voltage andthe second voltage may vary over time. Furthermore, in some cases theremay be a gap between application of the first voltage and the secondvoltage or between application of the second voltage and the firstvoltage, for example during which no voltage is applied to theelectrowetting element. In yet further examples, the first voltage maybe applied repeatedly or successively before the second voltage isapplied, and vice versa. As the skilled person will appreciate, thereare myriad ways in which the first and second voltages may be controlledor configured in order to, for example, obtain a configuration of thefirst fluid and the second fluid that corresponds with the first greylevel, for example with the first fluid moving continuously orsubstantially continuously.

FIGS. 5a to 5f show a further example series or sequence of fluidconfigurations that may be obtained with the electrowetting displaydevice 1 partly illustrated in FIG. 1.

In FIG. 5a , the first fluid 11 is in a retracted configuration with thefirst fluid at least partly in contact with the first portion 17 a butnot substantially in contact with the second portion 17 b. In thisexample, the retracted configuration of FIG. 5a is the same as thesecond configuration illustrated in FIG. 4b . For example, the controlsystem of the electrowetting display device 1 may have a control systemoperable to, in response to first input data indicative of a first greylevel, configure the first fluid in the retracted configuration of FIG.5a by application of what in this example is referred to as a fifthvoltage between the second fluid and the second electrode 17 b with afifth magnitude. In examples in accordance with FIG. 5a , with the fifthvoltage applied between the second fluid 12 and the second electrode 17b to configure the first fluid 11 in the retracted configuration, asixth voltage between the second fluid 12 and the first electrode 17 amay be applied with a sixth magnitude smaller than the fifth magnitudeof the fifth voltage, for example so that the electric field above thefirst portion is lower than the electric field above the second portion,so that the first fluid 11 retracts to contact the first portion withoutsubstantially contacting the second portion.

In response to input data indicative of a grey level, such as a secondgrey level, which in this example is a lighter grey level than the firstgrey level of FIGS. 4a and 4b , with the first fluid 11 more contractedfrom the surface of the electrowetting element 2, the fluidconfigurations of FIGS. 5b to 5f may be obtained.

FIG. 5b shows a fluid configuration which is the same as the firstconfiguration of FIG. 4a . In this example, the first fluidconfiguration of FIG. 5b is obtained by translating the first fluid 11from contact with the first portion (as shown in FIG. 5a ) to contact afirst extent or area of the second portion by application of a firstvoltage between the second fluid 12 and the first electrode 17 a. Inother words, the first fluid 11 may be considered to contact the secondportion over a first area in the first fluid configuration of FIG. 5b .In this example a first magnitude of the first voltage may be largerthan the magnitude of the first voltage of FIG. 4a as the second greylevel the electrowetting element 2 of FIGS. 5a to 5f is intended todisplay is lighter than the first grey level of FIGS. 4a and 4b .However, in other examples, the first magnitude of the first voltage fordisplay of the second grey level may be the same as or less than themagnitude of the first voltage for display of the first grey level. Withthe first voltage applied between the second fluid 12 and the firstelectrode 17 a to translate the first fluid 11, the control system maybe operable to apply a third voltage between the second fluid 12 and thesecond electrode 17 b with a third magnitude smaller than the firstmagnitude, for example to create a smaller electric field above thesecond electrode 17 b so that the first fluid 11 moves towards thesecond portion.

After the first voltage is applied between the first electrode 17 a andthe second fluid 12, a second voltage is applied between the secondelectrode 17 b and the second fluid 12. This is to retract the firstfluid 11 to contact a second extent or area of the second portion, thesecond extent smaller than the first extent, to obtain the first fluid11 configuration illustrated in FIG. 5c . In other words, the firstfluid 11 may be considered to contact the second portion over a secondarea in the first fluid configuration of FIG. 5c . Retraction of thefirst fluid 11 in this way typically uncovers part of the secondportion, so that the second extent is smaller than the first extent.With the second voltage applied between the second fluid 12 and thesecond electrode 17 b with a second magnitude to retract the first fluid11 as shown in FIG. 5c , the control system may be operable to apply afourth voltage between the second fluid 12 and the first electrode 17 awith a fourth magnitude substantially the same as or larger than thesecond magnitude, for example to prevent the first fluid 11 from flowingback to cover the first portion, due to the higher electric field abovethe first portion than above the second portion. In this example, thevoltage is maintained or increased between the first electrode 17 a andthe second fluid 12 while the second voltage is applied, for exampleeither by continuing to apply the voltage or by applying a voltage witha larger magnitude or due to the capacitance of the electrowettingelement 2. The second magnitude of the second voltage of FIG. 5c may belarger than the fifth magnitude of the fifth voltage to obtain theretracted configuration of the first fluid 11 illustrated in FIG. 5a .As the second grey level is a lighter grey level than the first greylevel of FIGS. 4a and 4b and the first grey level of FIG. 5a , thesecond voltage of FIG. 5c typically has a greater or larger magnitudethan the second voltage of FIG. 4 b.

With this sequence of voltages, the configuration of the first fluid andthe second fluid changes from the configuration of FIG. 5b to that ofFIG. 5c , in which the second fluid 12 adjoins or contacts the firstportion 18 a and part of the second portion 18 b and the first fluid 11contacts part of the second portion 18 b. In this example, theconfiguration of FIG. 5b may therefore be considered an intermediateconfiguration that is obtained as the first fluid 11 is furtherretracted or contracted to the configuration of FIG. 5c , which maycorrespond with a configuration associated with the second grey level.For example, the display effect obtained with the configuration of FIG.5b may be darker than the desired display effect to correspond to thesecond grey level. However, by applying the second voltage, the firstfluid 11 may continue to retract to the configuration of FIG. 5c , whichprovides the desired display state.

After obtaining the configuration of FIG. 5c , the first voltage betweenthe first electrode 17 a and the second fluid 12 and the second voltagebetween the second electrode 17 b and the second fluid 12 may be removedor reduced, allowing the first fluid 11 to relax or expand to theconfiguration illustrated in FIG. 5d . Subsequently to FIG. 5d , afurther voltage may be applied between the second electrode 17 b and thesecond fluid 12 to translate the first fluid 11 to contact the firstportion 18 a, as shown in FIG. 5e . The further voltage may have thesame magnitude as the first voltage of FIG. 5a . Subsequently, a yetfurther voltage may be applied between the first electrode 17 a and thesecond fluid 12 to retract the first fluid 11 to contact a smallerextent of the first portion 18 a in FIG. 5f than the extent of the firstportion 18 a contacted by the first fluid 11 in FIG. 5e . For example,the yet further voltage may have the same magnitude as the secondvoltage to retract the first fluid 11 to the configuration illustratedin FIG. 5c . In this way, both the configurations in FIGS. 5c and 5f maycorrespond to the desired display state.

In examples, the voltage sequence described with reference to FIGS. 5ato 5f may be repeatedly applied, for example similarly to the examplesdescribed above with reference to FIGS. 4a and 4b , to translate or movethe first fluid continuously back and forth across or over the surface14 of the electrowetting element 2.

In the examples described above, the first support plate 5 includes twoelectrodes: the first electrode 17 a and the second electrode 17 b.However, it is to be appreciated that the examples described above mayequally be extended to other examples in which the first support plateincludes more than two electrodes. For example, in addition to the firstelectrode 17 a and the second electrode 17 b described above, the firstsupport plate may also include a third electrode overlapped by a thirdportion of the surface, the third portion non-overlapping the firstportion and the second portion.

FIG. 6 shows an example in which the first support plate of an exampleelectrowetting element 202 includes a first electrode 217 a, a secondelectrode 217 b, a third electrode 217 c and a fourth electrode 217 d.Other than including four electrodes (and the necessary circuitry forcontrolling four electrodes) rather than two electrodes, the structureof the electrowetting element 202 of FIG. 6 is otherwise the same asthat of the electrowetting element 2 of FIGS. 1 and 2. Similar featuresare labelled with the same reference numerals but incremented by 200;corresponding descriptions should be taken to apply.

The example electrowetting element 202 of FIG. 6 is illustrated in planview. In this example, the first and second electrodes 217 a, 217 b areoverlapped by first and second portions of the surface of the firstsupport plate of the electrowetting element 202 similarly to theelectrowetting element 2 of FIG. 3. However, in this example, the firstand second portions of the surface do not alone make up the display area24. Instead, the display area 24, which corresponds to the surface 14 ofthe electrowetting element 2 bounded by the at least one wall 21 alsoincludes a third portion and a fourth portion. The third portion of thesurface overlaps the third electrode 217 c and the fourth portion of thesurface overlaps the fourth electrode 217 d. The third portion isnon-overlapping the first portion and the second portion and the fourthportion is non-overlapping the first portion, the second portion and thethird portion. In other words, the first, second, third and fourthportions may each not overlap or cover each other; they may each beseparate, distinct regions of the surface.

In this example, the first electrode 217 a, the second electrode 217 b,the third electrode 217 c and the fourth electrode 217 d are arranged ina row. For example, the first, second, third and fourth electrodes 217a, 217 b, 217 c, 217 d may be in the same row of a matrix or array ofelectrodes, for example corresponding to a matrix or array ofelectrowetting elements. Other electrode arrangements are possible inother examples though.

FIGS. 7a to 7g show example fluid configurations that may be obtainableby the electrowetting element 202 of FIG. 6 in response to input dataindicative of a first grey level. Prior to FIG. 7a , the electrowettingelement 202 is in the off state, which for example corresponds with adarkest grey level, with the first fluid 11 covering the surface of thefirst support plate. Then, the control system of the electrowettingelement 202 receives input data corresponding to or representing a firstgrey level, which may be a grey level other than the darkest grey level.

In response to the input data, the control system applies a firstvoltage between the first electrode 217 a and the second fluid toconfigure the first fluid and the second fluid in the firstconfiguration, which is shown in FIG. 7a . In this example, the secondfluid is in contact with the first portion of the surface and the firstfluid 211 is in contact with the second portion of the surface. Thefirst fluid 211 is also in contact with the third and fourth portions ofthe surface, as the first fluid 211 has retracted or contracted from theleft side of the electrowetting element 202 of FIG. 7a towards the rightside of the electrowetting element 202, to uncover the first portion ofthe first support plate.

However, in this example, the first grey level is a relatively lightgrey level, which is for example lighter than the first configurationillustrated in FIG. 7a . Accordingly, the first configuration of FIG. 7ais an intermediate configuration and the first fluid and the secondfluid continue to change configuration from that of the FIG. 7a to thesecond configuration shown in FIG. 7b . In FIG. 7b , a second voltage isapplied between the second electrode 217 b and the second fluid,subsequently to the application of the first voltage. In this example,in contrast to the example of FIGS. 4b and 5b , the first fluid is incontact with at least part of the third portion with the first fluid andthe second fluid in the second configuration. Although this exampleincludes four electrodes, it is to be appreciated that the first fluidmay also be in contact with at least part of the third portion with thefirst fluid and the second fluid in the second configuration in exampleelectrowetting elements with three rather than four electrodes.

In this example, the second configuration of FIG. 7b is also anintermediate configuration. Thus, in this example, and in response tothe input data indicative of the first grey level, the control system isoperable to apply, subsequently to the second voltage, a third voltagebetween the second fluid and the third electrode 217 c to contract thefirst fluid by configuring the first fluid and the second fluid in athird configuration with the second fluid in contact with at least partof the third portion. An example of the third configuration isillustrated in FIG. 7c . In this example, the second fluid is in contactwith the first portion, the second portion and the third portion and thefirst fluid 211 is in contact with at least part of (in this examplesubstantially all, for example more than 90% of) the first portion. Thethird configuration for example corresponds with the first grey leveland thus for example allows a proportion of light to be transmitted todisplay a display state corresponding to the first grey level.

The first, second and third voltages are applied sequentially, forexample with the second voltage applied after the first voltage and thethird voltage applied after the second voltage, in the example of FIGS.7a to 7c . This may improve the control of the retraction of the firstfluid 211, for example so that the first fluid 211 retracts from theleft of the electrowetting element 202 to the right side rather than theother way round or rather than the first fluid 211 retracting in twoopposite directions from a more central region of the surface. However,in other examples, as the skilled person will appreciate, the first,second and third voltages may be applied at the same time,simultaneously, or substantially simultaneously, such as simultaneouslywithin measurement errors, as each other.

In the example of FIGS. 7a to 7g , after the first fluid 211 isconfigured to adjoin or contact an extent of the first support platethat corresponds to the first grey level, the first fluid 211 may thensubsequently be translated across the surface, for example to obtainreciprocal motion of the first fluid 211 to improve the contrast ratioof the electrowetting element 202 as described above. The first fluid211 may be translated for example by sequentially or serially applyingvoltages between different ones of the first, second, third and fourthelectrodes 217 a, 217 b, 217 c, 217 d and the second fluid. For example,a voltage between the fourth electrode 217 d and the second fluid may beapplied to translate the first fluid 211 from the position illustratedin FIG. 7c to that shown in FIG. 7d , for example by configuring thefirst fluid and the second fluid in a fourth configuration with thesecond fluid in contact with at least part of the fourth portion.Subsequently, a voltage may be applied between the third electrode 217 cand the second fluid to move the first fluid 211 from contacting thethird portion to contacting the second portion, as shown in FIG. 7e .Similarly, a voltage may then be applied between the second electrode217 b and the second fluid to translate the first fluid 211 to theconfiguration of FIG. 7f , with the first fluid 211 in contact with thefirst portion, and then a voltage may be applied between the firstelectrode 217 a and the second fluid to move the first fluid 211 tocontact the second portion again, as shown in FIG. 7g . This sequence ofvoltages may be applied for example while the display effectcorresponding to the first grey level is to be shown or displayed by theelectrowetting element 202.

FIG. 8 shows a further example of an electrowetting element 302 in planview. The first support plate of the electrowetting element 302 of FIG.8 includes a first electrode 317 a, a second electrode 317 b, a thirdelectrode 317 c and a fourth electrode 317 d. The electrowetting element302 of FIG. 8 is the same as the example electrowetting element 202 ofFIG. 6 except for the spatial arrangement of the four electrodes.Accordingly, features of FIG. 8 similar to corresponding features ofFIG. 6 are labelled with the same reference numerals but incremented by100 compared with the reference numerals of FIG. 6; correspondingdescriptions should be taken to apply. In FIG. 6, the first, second,third and fourth electrodes 217 a, 217 b, 217 c, 217 d are arranged in arow. However, in FIG. 8, the first electrode 317 a and the fourthelectrode 317 d are arranged in a first row and the second electrode 317b and the third electrode 317 c are arranged in a second row adjacent tothe first row, with the first electrode 317 a and the second electrode317 b in a first column and the fourth electrode 317 d and the thirdelectrode 317 c in a second column adjacent to the first column.

FIGS. 9a to 9c show example fluid configurations that may be obtainableby the electrowetting element 302 of FIG. 8 in response to input dataindicative of a first grey level. Prior to FIG. 9a , the electrowettingelement 302 is in the off state, with the first fluid 311 covering thesurface of the first support plate. Then, the control system of theelectrowetting element 302 of FIG. 9a receives input data indicative ofa first grey level, which may be a grey level other than the darkestgrey level.

In response to the input data, the control system applies a firstvoltage between the first electrode 317 a and the second fluid, thensubsequently applies a second voltage between the second electrode 317 band the second fluid and then subsequently applies a third voltagebetween the third electrode 317 c and the second fluid, to configure thefirst fluid and the second fluid sequentially from the configuration ofFIG. 9a (with the first voltage applied), to that of FIG. 9b (with thesecond voltage applied), to that of FIG. 9c (with the third voltageapplied). In this way, the first fluid 311 may be gradually retractedfrom the surface until it reaches the third configuration, which in thisexample corresponds with the first grey level. By applying a series ofvoltages in this way, the motion of the first fluid 311 may beaccurately controlled, for example such that the first fluid 311retracts in a roughly circular fashion across the surface of the firstsupport plate.

After the configuration of FIG. 9c , the voltages between the secondfluid and one or more of the first, second, third and fourth electrodes317 a, 317 b, 317 c, 317 d may be removed or reduced, sequentially or atthe same or a similar time. In examples, this causes the configurationof the first fluid 311 to gradually reverse to the configuration of FIG.9a . In some cases, the first fluid 311 may continue to spread out orexpand from the configuration of FIG. 9a to contact a larger extent ofthe surface 14, for example a larger extent of the first portion 318 athan in FIG. 9a . For example, a higher voltage may be maintainedbetween the first electrode 317 a and the second fluid and between thesecond electrode 317 b and the second fluid than between the third andfourth electrodes 317 c, 317 d and the second fluid to cause the firstfluid to relax or expand from the configuration of FIG. 9c to that ofFIG. 9b . Subsequently, a higher voltage may be maintained between thefirst electrode 317 a and the second fluid than between the second,third and fourth electrodes 317 b, 317 c, 317 d to cause the first fluidto further expand from the configuration of FIG. 9b to that of FIG. 9a .Finally, the voltage between the first electrode 317 a and the secondfluid may be reduced to cause the first fluid to further expand toadjoin or contact a larger extent of the surface than in theconfiguration of FIG. 9a . With the first fluid 311 expanded in thisway, a dark grey level may be obtained, which may be darker than a greylevel obtainable with a known electrowetting element. For example, sucha dark grey level may be obtained by opening up the electrowettingelement 302 (by contracting the first fluid 311) as shown in FIGS. 9a to9c and then allowing the first fluid 311 to expand as described above.In this way, dark grey levels may be obtained without dithering orrepeatedly switching the electrowetting element from a darker state thana desired grey level and a lighter state than the desired grey level.This may therefore simplify the driving or controlling of theelectrowetting element and may reduce the appearance of displayartefacts due to dithering.

FIGS. 10a to 10f show further example fluid configurations that may beobtained using the electrowetting element 302 of FIG. 8 in response toinput data indicative of a second grey level, different from the firstgrey level. In this example, the second grey level is a lighter greylevel, in which the first fluid 311 is to be configured to adjoin orcontact a smaller extent of the surface than for the first grey level.In this example, a fourth voltage is applied between the second fluidand the fourth electrode 317 d to configure the first fluid and thesecond fluid in a fourth configuration with the second fluid in contactwith at least part of the fourth portion. FIG. 10a shows an intermediateconfiguration, in which the first fluid 311 is retracted to adjoin partof the fourth portion, before the first fluid 311 is translated tocontact the first portion in response to continued application of thefourth voltage, for example in conjunction with ceasing to apply avoltage between the first electrode 317 a and the second fluid. Thus,the first fluid 311 continues to move to the fourth configuration, whichis illustrated in FIG. 10b , with the first fluid 311 in contact withpart of the first portion.

Further in response to the input data indicative of the second greylevel, the control system in this example is operable to apply a fifthvoltage between the second fluid and the first electrode 317 a, amagnitude of the fifth voltage different from a magnitude of the firstvoltage, to configure the first fluid and the second fluid to a fifthconfiguration with the second fluid in contact with at least part of thefirst portion, the fifth configuration different from the firstconfiguration. FIG. 10c illustrates an example of the fifthconfiguration. As can be seen from a comparison of the firstconfiguration of FIG. 9a , the fifth configuration is indeed differentfrom the first configuration in this example. Moreover, the extent ofadjoinment or contact of the first fluid 311 with the surface of thefirst support plate with the configurations corresponding to the secondgrey level (illustrated in FIGS. 10a to 10f ) are different from thisextent with the configuration corresponding to the first grey level(illustrated in FIG. 9c ).

Similarly to the example of FIGS. 7c to 7g , a sequence of voltages maythen be applied between the second fluid and the first, second, thirdand fourth electrodes 317 a, 317 b, 317 c, 317 d to continue totranslate the first fluid 311 of FIGS. 10d to 10f across the surface ofthe first support plate. For example, a sequence of voltages such asthis may cause rotational motion of the first fluid 311 across the firstsurface. For example, the rotational motion may be substantially arounda center of rotation 56 located between a plurality of portions of thesurface, the plurality of portions including the first portion and thesecond portion, for example around the center of rotation withinmeasurement tolerances. For example, in FIGS. 10d to 10f , it can beseen that the first fluid 311 rotates, or moves in a roughly circulardirection, approximately or substantially around a center of rotation 56corresponding to the center of the electrowetting element 302, which isbetween the first, second, third and fourth electrodes 317 a, 317 b, 317c, 317 d. Rotational motion such as this may be considered to correspondto a vortex, or a region of the first fluid 311 in which the first fluid311 flow rotates around an axis, such as an axis corresponding to acenter of the surface. Rotational motion of the first fluid 311 may notinvolve movement of the entirety of the first fluid 311. For example, acentral portion of the first fluid 311, or a portion of the first fluid311 closest to the center of the surface, may sweep across the surface,drawing out a roughly circular path around a point on the surface suchas a central point. However, a peripheral portion of the first fluid311, such as a portion of the first fluid 311 towards an outer region ofthe surface, may be stationary or substantially stationary, for examplewith a speed of movement of less than 10% than the speed of movement ofthe central portion of the first fluid 311.

For example, the control system may be operable to apply a sequence ofvoltages to translate the first fluid from being at least partly incontact with the first portion (and in contact with or not substantiallyin contact with the second portion, the third portion and the fourthportion), to being at least partly in contact with the second portion(and in contact with or not substantially in contact with the firstportion, the third portion and the fourth portion), to, subsequently,being at least partly in contact with the third portion (and in contactwith or not substantially in contact with the first portion, the secondportion and the fourth portion), to, subsequently, being at least partlyin contact with the fourth portion (and in contact with or notsubstantially in contact with the first portion, the second portion andthe third portion), to, subsequently, being at least partly in contactwith the first portion (and in contact with or not substantially incontact with the second portion, the third portion and the fourthportion). As noted above, a fluid may be considered to be notsubstantially in contact with a portion of the surface where the fluidis not in contact with the portion entirely or within measurement errorsor is in contact with an insignificant or relatively small part of theportion such as less than 10% of the portion. For example, the firstfluid may be moved consecutively, with no intervening configurations,between at least partial contact with the first portion, the secondportion, the third portion, the fourth portion and then the firstportion again.

To translate the first fluid in this way, the sequence of voltages mayinclude a third voltage between the second fluid and the third electrode317 c and a fourth voltage between the second fluid and the fourthelectrode 317 d. For example, with the first voltage applied between thesecond fluid and the first electrode 317 a, the second voltage may havea smaller magnitude than the first voltage and both the third voltageand the fourth voltage may be substantially the same magnitude as (forexample, the same magnitude as or the same magnitude within measurementerrors as) or a larger magnitude than the first voltage. Similarly, withthe second voltage applied between the second fluid and the secondelectrode 317 b, the third voltage may have a smaller magnitude than thesecond voltage and both the first voltage and the fourth voltage may besubstantially the same magnitude as or a larger magnitude than thesecond voltage. Furthermore, with the third voltage applied between thesecond fluid and the third electrode 317 c, the fourth voltage may havea smaller magnitude than the third voltage and both the first voltageand the second voltage may be substantially the same magnitude as or alarger magnitude than the third voltage. Similarly, with the fourthvoltage applied between the second fluid and the fourth electrode 317 d,the first voltage may have a smaller magnitude than the fourth voltageand both the second voltage and the third voltage may be substantiallythe same magnitude as or a larger magnitude than the fourth voltage.

In this way, a region of lowest electric field may be moved sequentiallyaround the electrowetting element 302 from the second portion (with thefirst voltage applied), to the third portion (with the second voltageapplied), to the fourth portion (with the third voltage applied), to thefirst portion (with the fourth voltage applied). As the first fluid 311typically moves in a direction or towards a region of lowest electricfield, this causes the first fluid 311 to also move sequentially fromthe second portion, to the third portion, to the fourth portion and,then, to the first portion, causing rotational or roughly circularmotion of the first fluid 311 over the surface. Although not illustratedin FIGS. 10a to 10f it is to be appreciated that in examples similar toFIGS. 10a to 10f , the first fluid 311 may repeatedly expand andcontract as the first fluid 311 is moved from contacting the firstportion, to the second portion, to the third portion, to the fourthportion and back to the first portion. For example, the expansion andcontraction of the first fluid 311 may be similar to that illustrated inFIGS. 5a to 5f , with the first fluid 311 expanding from contacting lessthan all of the first portion to substantially cover the first portion,for example to cover at least 90% of the first portion, and subsequentlymoving to substantially cover the second portion, for example to coverat least 90% of the second portion, and subsequently retracting to covera smaller extent of the second portion, and so on as the first fluid 311moves across the surface.

FIG. 11 shows a plan view of part of an example electrowetting displaydevice 1′ including an electrowetting element 2′. Other than the shapeof the walls of the electrowetting element 2′ (and hence the shape ofthe display area) of FIG. 11, the electrowetting element 2′ is the sameas the electrowetting element 2 of FIG. 2. Hence, features of FIG. 11similar to corresponding features of FIG. 2 are labelled with the samereference numerals but with a prime (i.e. a ′); correspondingdescriptions are to be taken to apply.

In the example of FIG. 11, the at least one wall of the electrowettingelement 2′ includes a first wall portion, a second wall portion and athird wall portion which connects the first wall portion to the secondwall portion. The first wall portion and the second wall portion areeach substantially straight in a plane parallel to a plane of thesurface of the first support plate of the electrowetting element 2′(which is for example a hydrophobic surface). For example, asubstantially straight wall portion may be straight within manufacturingtolerances. The third wall portion is curved in the plane parallel tothe plane of the hydrophobic surface. For example, a curvature of thethird wall portion may be such that the diameter of a circle having asection corresponding to the curve of the third wall portion, such as acurve following or corresponding to an inner surface of the third wallportion, closest to the display area of the electrowetting element, isless than or equal to a width or half the width of the electrowettingelement 2. The third wall portion may be for example at least 10% of alength of the first wall portion or a length of the second wall portion.For example, the third wall portion may be sufficiently curved and/orsufficiently extended so as to aid flow of the first fluid over thesurface of the electrowetting element 2′, for example to reduce trappingor pinning of the first fluid in a corner of the electrowetting element2′.

Lines 28 a′, 28 b′ and 28 c′ in FIG. 11 indicate the inner border of thefirst wall portion, the second wall portion and the third wall portion,respectively, of the electrowetting element 2′. In the example of FIG.11, the first wall portion is orthogonal or at a right angle to thesecond wall portion. However, in other examples, the first and secondwall portions may be at a different angle with respect to each other.The curve of the third wall portion for example smoothly connects thefirst wall portion to the second wall portion, for example without adiscontinuous, sharp or jagged changed in direction of the wall betweenthe first wall portion and the second wall portion. The third wallportion may therefore correspond with a corner of the electrowettingelement, and may therefore provide a curved corner portion.

In the example of FIG. 11, there is a curved third wall portion at eachof the corners of the electrowetting element. Thus, in this example, thedisplay area, or the hydrophobic surface of the first support plate, hasthe shape of a rectangle with curved corners, where it is to beappreciated that a square is an example of a rectangle. In otherexamples, though, a subset of the corners of an electrowetting elementmay have a curved third wall portion, with other corners being sharprather than curved or rounded.

The curved third wall portion may provide a larger intersecting wallregion where the electrowetting element 2′ meets or adjoins aneighboring electrowetting element than an electrowetting elementwithout a curved third wall portion. For example, a known electrowettingelement may have a rectangular or square intersecting wall region, withthe first wall portion meeting the second wall portion at a right angle,with no curved portion between the first wall portion and the secondwall portion. However, the curved third wall portion typically increasesthe area of the intersecting wall region as the curved third wallportion is displaced in an inwards direction compared to a point atwhich the first wall portion would meet the second wall portion at aright angle.

By having a larger intersecting wall region, more robust spacer elementscan be located between the first support plate 5 and the second supportplate 6 (not shown in FIG. 11), for example with a larger cross-section.As the skilled person will appreciate, spacer elements are typicallyelongate elements that are used to prevent or reduce movement of thefirst and second support plates 5, 6 towards each other, for example inresponse to pressure on one or both of the first and second supportplates 5, 6. For example, a spacer element may extend from either one ofthe first and second support plates 5, 6 to the other one of the firstand second support plates 5, 6. With each spacer element being morerobust, the total number of spacers in an electrowetting display devicemay be reduced. This can therefore increase the aperture ratio of theelectrowetting display device, which may be reduced by the presence ofspacers.

As will be appreciated, other electrowetting elements with differentarrangements than that shown in FIG. 11 may also have a curved thirdwall portion such as that of FIG. 11. For example, the electrowettingelements 2, 202, 302 of FIGS. 2, 6 and 8 may also have one or morecurved third wall portions, which may aid or improve the motion of thefirst fluid in these electrowetting elements, for example by reducingthe separation of the first fluid into multiple separate portions orsatellites. For example, each of the corners of the electrowettingelements 2, 202, 302 of FIGS. 2, 6 and 8 may have a curved third wallportion, such that the display areas of the electrowetting elements 2,202, 302 are rectangular with curved corners. In examples in which anelectrowetting element comprises a plurality of curved third wallportions, a curvature of one of the third wall portions may differ fromthat of one or more of the other third wall portions. For example, eachthird wall portion may have a different curvature from each other thirdwall portion. It will further be appreciated that, in some cases,electrowetting elements such as those of FIGS. 2, 6 and 8 may not have acurved third wall portion.

FIGS. 12a to 12c illustrate further different fluid configurations thatmay be obtained using the example electrowetting element 2 of FIGS. 1and 2. FIGS. 12a to 12c show the same part of the electrowetting displaydevice 1 illustrated in FIG. 2, with corresponding elements labelledwith the same reference numerals. Some of the reference numeralsincluded in FIG. 2 are omitted in FIGS. 12a to 12c , for clarity. It isnoted that, although the structure of the electrowetting element 2 ofFIGS. 12a to 12c is the same as that of FIGS. 4a and 4b and FIGS. 5a to5f , the control system is differently configured.

In the example of FIGS. 12a to 12c , the control system is configured oroperable to receive first input data indicative of a first grey leveland, in response to the first input data, apply a first voltage betweenthe second fluid 12 and the second electrode 17 b to configure the firstfluid 11 in a retracted configuration with the first fluid 11 at leastpartly in contact with the first portion but not substantially incontact with the second portion. The control system is further operableto receive second input data indicative of a second grey level and, inresponse to the second input data, translate the first fluid 11 fromcontact with the first portion to contact the second portion byapplication of a second voltage between the second fluid 12 and thefirst electrode 17 a. For example, with the first fluid 11 in contactwith the second portion, the first fluid 11 may be absent from or maynot contact or may be substantially not in contact with the firstportion.

The first grey level is for example a different grey level than thesecond grey level. In the example of FIGS. 12a and 12b , the second greylevel is a lighter or brighter grey level than the first grey level.Hence, the first fluid 11 contacts a smaller extent of the surface withthe second voltage applied (FIG. 12b ) than with the first voltageapplied (FIG. 12a ).

In examples, the first voltage to translate the first fluid 11 fromcontact with the first portion to contact the second portion has a firstmagnitude and the second voltage has a second magnitude, which may bedifferent from the first magnitude. In such cases, the control systemmay be operable to receive third data indicative of a third grey leveldifferent from the first grey level and, in response to the third inputdata, translate the first fluid from contact with the second portion tocontact the first portion by application of a third voltage between thesecond fluid 12 and the first electrode 17 a with a third magnitudedifferent from the first magnitude, as shown in FIG. 12c . In theexample of FIG. 12c , the third grey level is lighter than the firstgrey level. Hence, the first fluid 11 adjoins a smaller extent of thefirst portion in FIG. 12c than in FIG. 12a . The control system may beoperable to apply the third voltage with the third magnitudeconsecutively after the second voltage, for example with no intermediateor intervening voltages between the second voltage and the thirdvoltage. In other cases, though, there may be other voltages between thesecond voltage and the third voltage. As will be appreciated, thevoltages described with reference to FIGS. 12a and 12c are merelyillustrative and in other examples, the voltages may be of differentmagnitudes, for example in cases where the second and/or the third greylevels are darker than the first grey level.

Thus, in examples such as FIGS. 12a to 12c , the control system may beconfigured to alternately switch the first fluid 11 from one side to anopposite side of an electrowetting element in response to different greylevels. This can improve the display quality, as the movement of thefirst fluid 11 back and forth across the surface of the electrowettingelement can improve coalescence of the first fluid 11 with portions orsatellites of the first fluid 11 that have separated from a main body offirst fluid 11. In contrast, in a known electrowetting element, thefirst fluid remains pinned or in constant adjoinment with one side ofthe electrowetting element, and therefore is less able to coalesce withsatellites of the first fluid 11, for example satellites at an oppositeside or edge of the electrowetting element.

Furthermore, the driving of the electrowetting element in this way mayprovide similar features to alternating current (AC) driving. Forexample, the movement of the first fluid 11 with the application of thevoltages described with reference to FIGS. 12a to 12c may be more rapidthan for a known electrowetting element. The electrowetting element ofthese examples may therefore have a better or quicker response to datarepresenting large and frequent changes of display effect compared witha known electrowetting element. Moreover, by reversing the direction ofmotion of the first fluid 11, backflow of the first fluid 11, in whichthe first fluid 11 tends to flow back to cover the surface despite anapplied voltage being maintained, may be reduced. Accordingly, resetpulses, which are typically applied to reduce backflow, may be omittedwhen using this method of driving. Furthermore, the driving in theseexamples may have a lower power consumption than alternating currentdriving.

In further examples in accordance with FIGS. 12a to 12c , the controlsystem is operable to receive fourth input data indicative of a fourthgrey level darker than the at least one of the first grey level or thesecond grey level. In response to the fourth input data, the controlsystem in these examples is operable to apply the first voltage betweenthe second fluid and the second electrode to configure the first fluidin a first retracted configuration with the first fluid at least partlyin contact with the first portion but not substantially in contact withthe second portion and, subsequently, expand the first fluid by removalor reduction of a magnitude of the first voltage such that the firstfluid at least partly contacts the first portion and the second portion.Alternatively, the control system in these examples may be operable toapply the second voltage between the second fluid and the firstelectrode to configure the first fluid in a second retractedconfiguration with the first fluid at least partly in contact with thesecond portion but not substantially in contact with the first portionand, subsequently, expand the first fluid by removal or reduction of amagnitude of the second voltage such that the first fluid at leastpartly contacts the first portion and the second portion. In this way,dark grey levels, for example darker than those obtainable with a knownelectrowetting element, may be obtained straightforwardly. The fourthinput data may for example be received after, for example consecutivelyor directly after, the second input data or the third input data.Typically, the first voltage will be applied in response to the fourthinput data in examples in which, prior to application of the firstvoltage, the first fluid is in contact with a larger extent of thesecond portion than the first portion. Conversely, the second voltagemay be applied in response to the fourth input data where, prior toapplication of the second voltage, the first fluid is in contact with alarger extent of the first portion than the second portion. In this way,the first fluid may be translated by application of the first voltage orthe second voltage rather than merely retracted.

As will be appreciated by the skilled person, the principles explainedwith reference to FIGS. 12a to 12c to obtain a relatively dark greylevel may also be applied to other electrowetting display devices orelectrowetting elements, such as those with different numbers orarrangements of electrodes. For example, as described above withreference to FIGS. 9a to 9c , a similar approach may be used to obtainrelatively dark grey levels in an electrowetting element with fourelectrodes.

FIG. 13 shows schematically a system diagram of an example system, forexample an apparatus 72, including an electrowetting display device, anelectrowetting element, an electrowetting display unit, or an array ofelectrowetting elements such as any of the examples described above. Theapparatus is for example a portable, for example mobile, device such asan electronic reader device such as a so-called “e-reader”, a tabletcomputing device, a laptop computing device, a mobile telecommunicationsdevice, a watch or a satellite navigation device; the apparatus mayalternatively be a display screen for installation in any machine ordevice requiring a display screen, for example a consumer appliance.

The system diagram illustrates an example of a basic hardwarearchitecture of the apparatus 72. The apparatus 72 includes at least oneprocessor 74 connected to and therefore in data communication with forexample: a display device control subsystem 76, which for example may beor include the circuitry 29 of the control system illustrated in FIG. 3,a communications subsystem 78, a user input subsystem 80, a powersubsystem 82 and system storage 84. The display device control subsystemis connected to and is therefore in data communication with theelectrowetting display device. The at least one processor 74 is forexample a general purpose processor, a microprocessor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,a discrete gate or transistor logic, discrete hardware components, orany suitable combination thereof designed to perform the functionsdescribed herein. A processor may also be implemented as a combinationof computing devices, e.g., a combination of a DSP and a microprocessor,a plurality of microprocessors, one or more microprocessors inconjunction with a DSP core, or any other such configuration. Theprocessor may be coupled, via one or more buses, to read informationfrom or write information to one or more memories, for example those ofthe system storage 84. The at least one processor may additionally, orin the alternative, contain memory, such as processor registers.

The display device control subsystem 76 for example includeselectrowetting display element driver components, for use in applying avoltage to any of the electrowetting elements, to address different suchdisplay elements. In examples the electrowetting elements are configuredaccording to an active matrix configuration and the display devicecontrol subsystem is configured to control switching elements such asthin film transistors (TFTs) of the apparatus 72 via circuitry tocontrol the electrowetting display elements. The circuitry may includesignal and control lines such as those described above, for example withreference to FIG. 3.

The communications subsystem 78 for example is configured for theapparatus to communicate with for example a computing device via a datanetwork, for example a computer network such as the Internet, a localarea network, a wide area network, a telecommunications network, a wirednetwork, a wireless network, or some other type of network. Thecommunications subsystem 78 may further for example include aninput/output (I/O) interface, such as a universal serial bus (USB)connection, a Bluetooth or infrared connection, or a data networkinterface for connecting the apparatus to a data network such as any ofthose described above. Content data as described later may betransferred to the apparatus via the communications subsystem.

The user input subsystem 80 may include for example an input device forreceiving input from a user of the apparatus. Example input devicesinclude, but are not limited to, a keyboard, a rollerball, buttons,keys, switches, a pointing device, a mouse, a joystick, a remotecontrol, an infrared detector, a voice recognition system, a bar codereader, a scanner, a video camera (possibly coupled with videoprocessing software to, e.g., detect hand gestures or facial gestures),a motion detector, a microphone (possibly coupled to audio processingsoftware to, e.g., detect voice commands), or other device capable oftransmitting information from a user to the device. The input device mayalso take the form of a touch-screen associated with the display device,in which case a user responds to prompts on the display device by touch.The user may enter textual information through the input device such asthe keyboard or the touch-screen.

The apparatus may also include a user output subsystem (not illustrated)including for example an output device for providing output to a user ofthe apparatus. Examples include, but are not limited to, a printingdevice, an audio output device including for example one or morespeakers, headphones, earphones, alarms, or haptic output devices. Theoutput device may be a connector port for connecting to one of the otheroutput devices described, such as earphones.

The power subsystem 82 for example includes power circuitry 86 for usein transferring and controlling power consumed by the apparatus. Thepower may be provided by a mains electricity supply or from a battery88, via the power circuitry. The power circuitry may further be used forcharging the battery from a mains electricity supply.

The system storage 84 includes at least one memory, for example at leastone of volatile memory 90 and non-volatile memory 92 and may include anon-transitory computer readable storage medium. The volatile memory mayfor example be a Random Access Memory (RAM). The non-volatile (NV)memory may for example be a solid state drive (SSD) such as Flashmemory, or Read Only Memory (ROM). Further storage technologies may beused, for example magnetic, optical or tape media, compact disc (CD),digital versatile disc (DVD), Blu-ray or other data storage media. Thevolatile and/or non-volatile memory may be removable or non-removable.

Any of the memories may store data for controlling the apparatus, forexample components or subsystems of the apparatus. Such data may forexample be in the form of computer readable and/or executableinstructions, for example computer program instructions. Therefore, theat least one memory and the computer program instructions may beconfigured to, with the at least one processor, control a display effector grey level provided by the electrowetting display device.

In the example of FIG. 13, the volatile memory 90 stores for exampledisplay device data 94 which is indicative of display effects or greylevels to be provided by the electrowetting display device. The at leastone processor 74 may transmit data, based on the display device data, tothe display device control subsystem 76 which in turn outputs signals tothe electrowetting display device for applying voltages to theelectrowetting elements, for providing display effects or grey levelsfrom the electrowetting display device.

The non-volatile memory 92 stores for example program data 96 and/orcontent data 98. The program data 96 is for example data representingcomputer executable instructions, for example in the form of computersoftware, for the apparatus to run applications or program modules forthe apparatus or components or subsystems of the apparatus to performcertain functions or tasks, and/or for controlling components orsubsystems of the apparatus. For example, application or program moduledata includes any of routines, programs, objects, components, datastructures or similar. The content data 98 is for example datarepresenting content for example for a user; such content may representany form of media, for example text, at least one image or a partthereof, at least one video or a part thereof, at least one sound ormusic or a part thereof. Data representing an image or a part thereof isfor example representative of a display effect or grey level to beprovided by at least one electrowetting element of the electrowettingdisplay device. The content data may include data representing a libraryof content, for example a library of any of books, periodicals,newspapers, movies, videos, music, or podcasts, each of which may berepresented by a collection of data which represents for example onebook or one movie. Such a collection of data may include content data ofone type, but may instead include a mixture of content data of differenttypes, for example a movie may be represented by data including at leastimage data and sound data.

The above examples are to be understood as illustrative examples.Further examples are envisaged.

For example, the description above refers to a control system configuredto or at least one memory and computer program instructions configuredto, with at least one processor, apply voltages to an electrowettingelement. It is to be appreciated that application of such voltages mayinvolve determining a voltage with a particular magnitude, generatingthe voltage with the particular magnitude and transmitting the voltagewith the particular magnitude to the electrowetting element forapplication, which may be performed by the control system or by theleast one memory and the computer program instructions, with the atleast one processor.

It is to be understood that any feature described in relation to any oneexample may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the examples, or any combination of any other of theexamples. Furthermore, equivalents and modifications not described abovemay also be employed without departing from the scope of theaccompanying claims.

What is claimed is:
 1. An electrowetting display device comprising: anelectrowetting element comprising: a first fluid; a second fluidimmiscible with the first fluid; a first support plate having a firstsurface and a second surface opposite to the first surface, the firstsupport plate comprising: at least one wall corresponding to a perimeterof the first surface; a first electrode overlapped by a first portion ofthe first surface; and a second electrode overlapped by a second portionof the first surface, with the first portion and the second portionnon-overlapping each other; and a second support plate, the first fluidand the second fluid located between the first support plate and thesecond support plate; and a control system operable to, in response toinput data indicative of a first grey level: apply a first voltagebetween the second fluid and the first electrode such that the secondfluid is in contact with at least part of the first portion and is in afirst configuration corresponding to the first grey level; and,subsequently, apply a second voltage between the second fluid and thesecond electrode to translate the first fluid such that the second fluidis in contact with at least part of the second portion and is in asecond configuration corresponding to the first grey level, the secondconfiguration different from the first configuration.
 2. Theelectrowetting display device according to claim 1, wherein theelectrowetting element comprises a color filter overlapping at least thefirst electrode and the second electrode.
 3. The electrowetting displaydevice according to claim 1, wherein the control system is operable to,during display of the first grey level, apply a sequence of voltagescomprising the first voltage and the second voltage to translate thefirst fluid substantially continuously across the first surface.
 4. Theelectrowetting display device according to claim 1, wherein the controlsystem is operable to, during display of the first grey level, apply asequence of voltages comprising the first voltage and the second voltageto: cause rotational motion of the first fluid across the first surface,the rotational motion being substantially around a center of rotation,or cause reciprocating motion of the first fluid across the firstsurface.
 5. The electrowetting display device according to claim 1,wherein the control system is operable to, in response to the input dataindicative of the first grey level, apply a sequence of voltagescomprising repeated application of: the first voltage between the secondfluid and the first electrode; and, subsequently, the second voltagebetween the second fluid and the second electrode.
 6. The electrowettingdisplay device according to claim 1, wherein the control system isoperable to cease application of the first voltage before or atsubstantially the same time as application of the second voltage.
 7. Theelectrowetting display device according to claim 1, wherein the firstsupport plate comprises: a third electrode overlapped by a third portionof the first surface; and a fourth electrode overlapped by a fourthportion of the first surface, with the first portion, the secondportion, the third portion and the fourth portion non-overlapping eachother, wherein, with the first voltage applied, the first fluid is incontact with at least part of the second portion and, with the secondvoltage applied, the first fluid is in contact with at least part of thethird portion.
 8. The electrowetting display device according to claim7, wherein the control system is operable to, in response to the inputdata indicative of the first grey level: apply a third voltage betweenthe second fluid and the third electrode, subsequently to the secondvoltage, to translate the first fluid such that the second fluid is incontact with at least part of the third portion; and, subsequently,apply a fourth voltage between the second fluid and the fourth electrodeto translate the first fluid such that the second fluid is in contactwith at least part of the fourth portion.
 9. The electrowetting displaydevice according to claim 1, wherein the control system is operable to,during display of the first grey level, apply a sequence of voltagescomprising the first voltage and the second voltage to translate thefirst fluid across the first surface with a substantially constantextent of contact between the first fluid and the first surface.
 10. Adisplay apparatus comprising: an electrowetting element comprising: afirst fluid; a second fluid immiscible with the first fluid; a firstsupport plate comprising: a hydrophobic surface; at least one wallcorresponding to a perimeter of the hydrophobic surface; a firstelectrode overlapped by a first portion of the hydrophobic surface; anda second electrode overlapped by a second portion of the hydrophobicsurface, with the first portion and the second portion non-overlappingeach other; and a second support plate, the first fluid and the secondfluid located between the first support plate and the second supportplate; at least one processor; and at least one memory comprisingcomputer program instructions, the at least one memory and the computerprogram instructions operable to, with the at least one processor:receive input data indicative of a grey level; in response to the inputdata: apply a sequence of voltages between the second fluid and thefirst electrode and between the second fluid and the second electrode totranslate the first fluid across the hydrophobic surface such that: thefirst fluid is in contact with at least part of the first portion at afirst time and is in a first configuration corresponding to the greylevel; the first fluid is in contact with at least part of the secondportion at a second time subsequent to the first time and is in a secondconfiguration corresponding to the grey level, the second configurationdifferent from the first configuration; and the first fluid is incontact with the at least part of the first portion at a third timesubsequent to the first time and the second time.
 11. The displayapparatus according to claim 10, wherein the electrowetting elementcomprises a color filter overlapping at least the first electrode andthe second electrode.
 12. The display apparatus according to claim 10,wherein the at least one memory and the computer program instructionsare operable to, with the at least one processor, apply the sequence ofvoltages to translate the first fluid substantially continuously acrossthe hydrophobic surface during display of the grey level.
 13. Thedisplay apparatus according to claim 10, wherein the at least one memoryand the computer program instructions are operable to, with the at leastone processor, apply the sequence of voltages to: cause rotationalmotion of the first fluid across the surface, the rotational motionbeing substantially around a center of rotation, or cause reciprocatingmotion of the first fluid across the surface.
 14. The display apparatusaccording to claim 10, wherein the sequence of voltages comprisesrepeated application of: a first voltage between the second fluid andthe first electrode; and, subsequently, a second voltage between thesecond fluid and the second electrode.
 15. The display apparatusaccording to claim 14, wherein the at least one memory and the computerprogram instructions are operable to, with the at least one processor,cease application of the first voltage before or at substantially thesame time as application of the second voltage.
 16. An electrowettingdisplay device comprising: an electrowetting element comprising: a firstfluid; a second fluid immiscible with the first fluid; a first supportplate having a hydrophobic surface, the first support plate comprising:at least one wall corresponding to a perimeter of the hydrophobicsurface, the at least one wall comprising: a first wall portion, thefirst wall portion being substantially straight in a plane parallel to aplane of the hydrophobic surface; a second wall portion, the second wallportion being substantially straight in the plane parallel to the planeof the hydrophobic surface; and a third wall portion which connects thefirst wall portion to the second wall portion, the third wall portionbeing curved in the plane parallel to the plane of the hydrophobicsurface; a first electrode overlapped by a first portion of thehydrophobic surface; and a second electrode overlapped by a secondportion of the hydrophobic surface, with the first portion and thesecond portion non-overlapping each other; and a second support plate,the first fluid and the second fluid located between the first supportplate and the second support plate; an additional electrode; and acontrol system operable to, in response to input data indicative of afirst grey level: apply a first voltage between the additional electrodeand the first electrode such that the second fluid is in contact with atleast part of the first portion and is in a first configurationcorresponding to the first grey level; and, subsequently, apply a secondvoltage between the additional electrode and the second electrode totranslate the first fluid such that the second fluid is in contact withat least part of the second portion and is in a second configurationcorresponding to the first grey level, the second configurationdifferent from the first configuration.
 17. The electrowetting displaydevice according to claim 16, wherein the control system is operable to:cease application of the first voltage before or at substantially thesame time as application of the second voltage.
 18. The electrowettingdisplay device according to claim 16, comprising a plurality of theelectrowetting element, wherein a first distance between the firstelectrode and the second electrode of a first one of the plurality ofthe electrowetting element is smaller than a second distance between thefirst electrode of the first one of the plurality of the electrowettingelement and the first electrode of a second one of the plurality of theelectrowetting element, the first electrode of the first one of theplurality of the electrowetting element neighboring the first electrodeof the second one of the plurality of the electrowetting element. 19.The electrowetting display device according to claim 18, wherein atleast one of: the first electrode of the first one of the plurality ofthe electrowetting element, the second electrode of the first one of theplurality of the electrowetting element, the first electrode of thesecond one of the plurality of the electrowetting element, or the secondelectrode of the second one of the plurality of the electrowettingelement is reflective for light of at least one wavelength.
 20. Theelectrowetting display device according to claim 16, wherein theelectrowetting element comprises a color filter overlapping at least thefirst electrode and the second electrode.